DevTools: Show preview in console of ES6 Map, Set, WeakMap and WeakSet entries.

Source/devtools/front_end/components/ObjectPropertiesSection.js

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 560 matching lines @@
         treeElement.appendChild(new WebInspector.CollectionEntriesMainTreeElement(value));
     if (internalProperties) {
         for (var i = 0; i < internalProperties.length; i++) {
             internalProperties[i].parentObject = value;
             treeElement.appendChild(new treeElementConstructor(internalProperties[i]));
         }
     }
 }
 
 /**
- * @param {!WebInspector.RemoteObject} object
+ * @param {?WebInspector.RemoteObject} object
  * @param {!Array.<string>} propertyPath
  * @param {function(?WebInspector.RemoteObject, boolean=)} callback
  * @return {!Element}
  */
 WebInspector.ObjectPropertyTreeElement.createRemoteObjectAccessorPropertySpan = function(object, propertyPath, callback)
 {
     var rootElement = document.createElement("span");
-    var element = rootElement.createChild("span", "properties-calculate-value-button");
+    var element = rootElement.createChild("span");
     element.textContent = WebInspector.UIString("(...)");
+    if (!object)
+        return rootElement;
+    element.classList.add("properties-calculate-value-button");
     element.title = WebInspector.UIString("Invoke property getter");
     element.addEventListener("click", onInvokeGetterClick, false);
 
     function onInvokeGetterClick(event)
     {
         event.consume();
         object.getProperty(propertyPath, callback);
     }
 
     return rootElement;
@@ skipping 456 matching lines @@
 {
     WebInspector.TextPrompt.call(this, WebInspector.ExecutionContextSelector.completionsForTextPromptInCurrentContext);
     this.setSuggestBoxEnabled(true);
     if (renderAsBlock)
         this.renderAsBlock();
 }
 
 WebInspector.ObjectPropertyPrompt.prototype = {
     __proto__: WebInspector.TextPrompt.prototype
 }