Fix a bug: observatory debugger can't expand instances.

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

Index: runtime/observatory/lib/src/elements/debugger.dart
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index a01064fb9e18b43ce6e05f7f253b218a5133879e..b3ca5693fd3b00e7b831eb127e6e7b95524bbda6 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -1969,6 +1969,7 @@ class DebuggerPageElement extends HtmlElement implements Renderable {
     final e = document.createElement(tag.name);
     final debugger = new ObservatoryDebugger(isolate);
     debugger.page = e;
+    debugger.instances = instances;
     e._isolate = isolate;
     e._debugger = debugger;
     e._instances = instances;