Fix many warnings/errors when building Observatory

runtime/observatory/tests/observatory_ui/mocks/objects/vm.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 VMRefMock implements M.VMRef {
   final String name;
   final String displayName;
   const VMRefMock({this.name: 'vm-name', this.displayName: 'vm-display-name'});
 }
 
 class VMMock implements M.VM {
   final String name;
   final String displayName;
   final int architectureBits;
   final String targetCPU;
   final String hostCPU;
   final String version;
   final int pid;
   final int maxRSS;
   final DateTime startTime;
   final Iterable<M.IsolateRef> isolates;
+  final int nativeZoneMemoryUsage = 0;
+  final int heapAllocatedMemoryUsage = 0;
+  final int heapAllocationCount = 0;
+
+  Future<dynamic> invokeRpc(String method, Map params) {
+    return null;
+  }
 
   const VMMock(
       {this.name: 'vm-name',
       this.displayName: 'vm-display-name',
       this.architectureBits,
       this.targetCPU,
       this.hostCPU,
       this.version,
       this.pid: 0,
       this.maxRSS: 0,
       this.startTime,
       this.isolates: const []});
 }