Address observatory analysis issues.

runtime/observatory/tests/observatory_ui/mocks/objects/isolate.dart

 // Copyright (c) 2016, 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.
 
 part of mocks;
 
 class IsolateRefMock implements M.IsolateRef {
   final String id;
   final int number;
   final String name;
@@ skipping 35 matching lines @@
       this.libraries: const [],
       this.error,
       this.extensionRPCs: const [],
       this.counters: const {},
       this.newSpace: const HeapSpaceMock(),
       this.oldSpace: const HeapSpaceMock(),
       this.status: M.IsolateStatus.loading,
       this.pauseEvent,
       this.rootLibrary,
       this.entry});
+
+  Future collectAllGarbage() async {
+    throw "Unimplemented";
+  }
 }