The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    Ext.ns('Ext.debug');
    Ext.debug.Assistant = function(){
        var enabled = true;

        return {
            enable: function(){
                enabled = true;
            },

            disable: function(){
                enabled = false;
            },

            init : function(classes){
                var klass,
                    intercept = false,
                    fn,
                    method;
                Ext.each(classes, function(cls){
                    if(this.namespaceExists(cls.name)){
                        klass = this.getClass(cls.name);
                        method = cls.instance ? this.addInstanceCheck : this.addPrototypeCheck;
                        Ext.each(cls.checks, function(check){
                            intercept = check.intercept == true;
                            fn = method.call(this, klass, check.name, check.fn, check.intercept == true);
                            if(check.after){
                                check.after(fn);
                            }
                        }, this);
                    }
                }, this);
            },

            namespaceExists: function(name){
                var parent = window,
                    exists = true;

                Ext.each(name.split('.'), function(n){
                    if(!Ext.isDefined(parent[n])){
                        exists = false;
                        return false;
                    }
                    parent = parent[n];
                });
                return exists;
            },

            getClass : function(name){
                var parent = window;
                Ext.each(name.split('.'), function(n){
                    parent = parent[n];
                });
                return parent;
            },

            warn: function(){
                if(enabled && window.console){
                    console.warn.apply(console, arguments);
                }
            },

            error: function(){
                if(enabled && window.console){
                    console.error.apply(console, arguments);
                }
            },

            addPrototypeCheck : function(cls, method, fn, intercept){
                return (cls.prototype[method] = cls.prototype[method][intercept ? 'createInterceptor' : 'createSequence'](fn));
            },

            addInstanceCheck : function(cls, method, fn, intercept){
                return (cls[method] = cls[method][intercept ? 'createInterceptor' : 'createSequence'](fn));
            }
        };
    }();

    (function(){
        var A = Ext.debug.Assistant,
            cls = [];

        cls.push({
            name: 'Ext.util.Observable',
            checks: [{
                name: 'addListener',
                intercept: true,
                fn: function(eventName, fn){
                    if(typeof eventName == 'object'){
                        var ev, o;
                        for(ev in eventName){
                            if(!this.filterOptRe.test(ev)){
                                o = eventName[ev];
                                o = o && o.fn ? o.fn : o;
                                if(!Ext.isFunction(o)){
                                    A.error('Non function passed to event listener', this, ev);
                                    return false;
                                }
                            }
                        }
                    }else{
                        if(!Ext.isFunction(fn)){
                            A.error('Non function passed to event listener', this, eventName);
                        }
                    }
                },
                after: function(method){
                    Ext.util.Observable.prototype.on = method;
                }
            }]
        });

        cls.push({
            name: 'Ext.Component',
            checks: [{
                name: 'render',
                intercept: true,
                fn: function(container, position){
                    if(!container && !this.el){
                        A.error('Unable to render to container', this, container);
                    }

                    if(this.contentEl){
                        var el = Ext.getDom(this.contentEl);
                        if(!el){
                            A.error('Specified contentEl does not exist', this, this.contentEl);
                            return false;
                        }
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.Container',
            checks: [{
                name: 'onBeforeAdd',
                intercept: true,
                fn: function(c){
                    if(c.isDestroyed){
                        A.warn('Adding destroyed component to container', c, this);
                    }
                    if(c.renderTo){
                        A.warn('Using renderTo while adding an item to a Container. You should use the add() method or put the item in the items configuration', c, this);
                    }
                    if(c.applyTo){
                        A.warn('Using applyTo while adding an item to a Container. You should use the add() method or put the item in the items configuration', c, this);
                    }

                    var type = this.layout.type;
                    if(type == 'container' || type == 'auto'){
                        A.warn('A non sizing layout is being used in a container that has child components. This means the child components will not be sized.', this);
                    }
                }
            },{
                name: 'lookupComponent',
                intercept: true,
                fn: function(c){
                    var valid = true;
                    if(Ext.isEmpty(c)){
                        valid = false;
                    }
                    if(Ext.isString(c)){
                        c = Ext.ComponentMgr.get(comp);
                        valid = !Ext.isEmpty(c);
                    }
                    if(!valid){
                        A.error('Adding invalid component to container', this, c);
                        return false;
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.DataView',
            checks: [{
                name: 'initComponent',
                fn: function(){
                    if(!this.itemSelector){
                        A.error('No itemSelector specified', this);
                    }
                }
            },{
                name: 'afterRender',
                fn: function(){
                    if(!this.store){
                        A.error('No store attached to DataView', this);
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.Window',
            checks: [{
                name: 'show',
                intercept: true,
                fn: function(){
                    if(this.isDestroyed){
                        A.error('Trying to show a destroyed window. If you want to reuse the window, look at the closeAction configuration.', this);
                        return false;
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.grid.GridPanel',
            checks: [{
                name: 'initComponent',
                fn: function(){
                    if(!this.colModel){
                        A.error('No column model specified for grid', this);
                    }
                    if(!this.store){
                        A.error('No store specified for grid', this);
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.grid.GridView',
            checks: [{
                name: 'autoExpand',
                intercept: true,
                fn: function(){
                    var g = this.grid,
                    cm = this.cm;
                    if(!this.userResized && g.autoExpandColumn){
                        var tw = cm.getTotalWidth(false),
                            aw = this.grid.getGridEl().getWidth(true) - this.getScrollOffset();
                        if(tw != aw){
                            var ci = cm.getIndexById(g.autoExpandColumn);
                            if(ci == -1){
                                A.error('The autoExpandColumn does not exist in the column model', g, g.autoExpandColumn);
                                return false;
                            }
                        }
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.chart.Chart',
            checks: [{
                name: 'initComponent',
                fn: function(){
                    if(!this.store){
                        A.error('No store specified for chart', this);
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.tree.TreePanel',
            checks: [{
                name: 'afterRender',
                intercept: true,
                fn: function(){
                    if(!this.root){
                        A.error('No root node specified for tree', this);
                        return false;
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext',
            instance: true,
            checks: [{
                name: 'extend',
                intercept: true,
                fn: function(){
                    if(arguments.length == 2 && !arguments[0]){
                        A.error('Invalid base class passed to extend', arguments[0]);
                        return false;
                    }
                    if(arguments.length == 3){
                        if(!arguments[0]){
                            A.error('Invalid class to extend', arguments[0]);
                            return false;
                        }else if(!arguments[1]){
                            A.error('Invalid base class passed to extend', arguments[1]);
                            return false;
                        }
                    }
                }
            },{
                name: 'override',
                intercept: true,
                fn: function(c){
                    if(!c){
                        A.error('Invalid class passed to override', c);
                        return false;
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.ComponentMgr',
            instance: true,
            checks: [{
                name: 'register',
                intercept: true,
                fn: function(c){
                    if(this.all.indexOfKey(c.id) > -1){
                        A.warn('A component with this id already exists', c, c.id);
                    }
                }
            },{
                name: 'create',
                intercept: true,
                fn: function(config, defaultType){
                    var types = Ext.ComponentMgr.types;
                    if(!config.render){
                        if(config.xtype){
                            if(!types[config.xtype]){
                                A.error('Unknown xtype specified', config, config.xtype);
                                return false;
                            }
                        }else{
                            if(!types[defaultType]){
                                A.error('Unknown defaultType specified', config, defaultType);
                                return false;
                            }
                        }
                    }
                }
            }]
        });

        cls.push({
            name: 'Ext.layout.FitLayout',
            checks: [{
                name: 'onLayout',
                intercept: true,
                fn: function(){
                    var ct = this.container;
                    if(ct.items.getCount() > 1){
                        A.warn('More than 1 item in the container. A fit layout will only display a single item.', ct);
                    }
                }
            }]
        });

        if(Ext.BLANK_IMAGE_URL == 'http:/' + '/www.extjs.com/s.gif'){
            A.warn('You should set the Ext.BLANK_IMAGE_URL to reference a local copy.');
        }

        A.init(cls);

    })();