Reapply "[vm] Don't enable the profiler by default. Enable the profiler at startup with --observe, …

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'});
@@ skipping 11 matching lines @@
   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;
   }
 
+  Future enableProfiler() {
+    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 []});
 }