Fix many warnings/errors when building Observatory

runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.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 HeapSnapshotMock implements M.HeapSnapshot {
   final DateTime timestamp;
   final int objects;
   final int references;
   final int size;
   final M.HeapSnapshotDominatorNode dominatorTree;
+  final M.HeapSnapshotMergedDominatorNode mergedDominatorTree = null;
   final Iterable<M.HeapSnapshotClassReferences> classReferences;
 
   const HeapSnapshotMock(
       {this.timestamp,
       this.objects: 0,
       this.references: 0,
       this.size: 0,
       this.dominatorTree: const HeapSnapshotDominatorNodeMock(),
       this.classReferences: const []});
 }
 
 class HeapSnapshotDominatorNodeMock implements M.HeapSnapshotDominatorNode {
   final int shallowSize;
   final int retainedSize;
+  final bool isStack = false;
   final Future<M.ObjectRef> object;
   final Iterable<M.HeapSnapshotDominatorNode> children;
 
   const HeapSnapshotDominatorNodeMock(
       {this.shallowSize: 1,
       this.retainedSize: 1,
       this.object,
       this.children: const []});
 }
 
@@ skipping 31 matching lines @@
   final M.ClassRef target;
   final int count;
   final int shallowSize;
   final int retainedSize;
   const HeapSnapshotClassOutboundMock(
       {this.target: const ClassRefMock(),
       this.count: 1,
       this.shallowSize: 1,
       this.retainedSize: 2});
 }