Improve Code object support in service and observatory

runtime/bin/vmservice/client/lib/src/observatory_elements/disassembly_entry.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library disassembly_entry_element;
 
+import 'package:observatory/observatory.dart';
 import 'package:polymer/polymer.dart';
 import 'observatory_element.dart';
 
 @CustomTag('disassembly-entry')
 class DisassemblyEntryElement extends ObservatoryElement {
-  @published Map instruction = toObservable({});
+  @published CodeInstruction instruction;
   DisassemblyEntryElement.created() : super.created();
 }