Cleanup some Observatory errors caught by dart2js

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 dc5f4439aa4f2a4c073c870d7cf807ee29a6c8a1..8c817e558ca7654289e0253479038c6f65a1c5bc 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -2048,6 +2048,11 @@ class DebuggerPageElement  extends HtmlElement implements Renderable {
   }
 
   @override
+  void render() {
+    /* nothing to do */
+  }
+
+  @override
   void detached() {
     _timer.cancel();
     children = const [];
@@ -2168,6 +2173,10 @@ class DebuggerStackElement extends HtmlElement implements Renderable {
     return e;
   }
 
+  void render() {
+    /* nothing to do */
+  }
+
   _addFrame(List frameList, S.Frame frameInfo) {
     final frameElement = new DebuggerFrameElement(_isolate,
                                                   frameInfo,
@@ -2926,6 +2935,10 @@ class DebuggerConsoleElement extends HtmlElement implements Renderable {
     children.clear();
   }
 
+  void render() {
+    /* nothing to do */
+  }
+
   ObservatoryApplication get app => ObservatoryApplication.app;
 }
 
@@ -3100,6 +3113,10 @@ class DebuggerInputElement extends HtmlElement implements Renderable {
   void focus() {
     _textBox.focus();
   }
+
+  void render() {
+    // Nothing to do.
+  }
 }
 
 final SvgSvgElement iconExpandLess = new SvgSvgElement()