Fixed missing method referenced when MallocHooks is unsupported.

runtime/observatory/lib/src/elements/class_ref.dart

Index: runtime/observatory/lib/src/elements/class_ref.dart
diff --git a/runtime/observatory/lib/src/elements/class_ref.dart b/runtime/observatory/lib/src/elements/class_ref.dart
index 21bd6cce2f3798c4494584a9bfbca87c1f53c55f..1d0ae508ce9e675cfe84bf658841b9d9888a1a48 100644
--- a/runtime/observatory/lib/src/elements/class_ref.dart
+++ b/runtime/observatory/lib/src/elements/class_ref.dart
@@ -24,7 +24,6 @@ class ClassRefElement extends HtmlElement implements Renderable {
 
   factory ClassRefElement(M.IsolateRef isolate, M.ClassRef cls,
       {RenderingQueue queue}) {
-    assert(isolate != null);
     assert(cls != null);
     ClassRefElement e = document.createElement(tag.name);
     e._r = new RenderingScheduler(e, queue: queue);
@@ -50,8 +49,10 @@ class ClassRefElement extends HtmlElement implements Renderable {
 
   void render() {
     children = [
-      new AnchorElement(href: Uris.inspect(_isolate, object: _class))
-        ..text = _class.name
+      new AnchorElement(
+          href: (_isolate == null)
+              ? null
+              : Uris.inspect(_isolate, object: _class))..text = _class.name
     ];
   }
 }