Fix many warnings/errors when building Observatory

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 10 matching lines @@
   final Iterable<M.LibraryRef> libraries;
   final M.Error error;
   final Iterable<String> extensionRPCs;
   final Map counters;
   final M.HeapSpace newSpace;
   final M.HeapSpace oldSpace;
   final M.IsolateStatus status;
   final M.DebugEvent pauseEvent;
   final M.LibraryRef rootLibrary;
   final M.FunctionRef entry;
+  final Iterable<M.Thread> threads = null;
+  final int zoneHighWatermark = 0;
+  final int numZoneHandles = 0;
+  final int numScopedHandles = 0;
 
   const IsolateMock(
       {this.id: 'i-id',
       this.number,
       this.name: 'i-name',
       this.startTime,
       this.runnable: true,
       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});
 }