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

Source/devtools/front_end/components/ObjectPropertiesSection.js

Index: Source/devtools/front_end/components/ObjectPropertiesSection.js
diff --git a/Source/devtools/front_end/components/ObjectPropertiesSection.js b/Source/devtools/front_end/components/ObjectPropertiesSection.js
index 6512bf7146235c4661b6e96239cb4954060a7cbb..ed1f0a52d61b53d48a219c989aef4772321ffa1e 100644
--- a/Source/devtools/front_end/components/ObjectPropertiesSection.js
+++ b/Source/devtools/front_end/components/ObjectPropertiesSection.js
@@ -578,7 +578,7 @@ WebInspector.ObjectPropertyTreeElement.populateWithProperties = function(treeEle
 }
 
 /**
- * @param {!WebInspector.RemoteObject} object
+ * @param {?WebInspector.RemoteObject} object
  * @param {!Array.<string>} propertyPath
  * @param {function(?WebInspector.RemoteObject, boolean=)} callback
  * @return {!Element}
@@ -586,8 +586,11 @@ WebInspector.ObjectPropertyTreeElement.populateWithProperties = function(treeEle
 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);