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

runtime/observatory/lib/src/models/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 models;
 
 abstract class VMRef {
   /// A name identifying this vm. Not guaranteed to be unique.
   String get name;
 
@@ skipping 33 matching lines @@
 
   int get maxRSS;
 
   /// The time that the VM started in milliseconds since the epoch.
   ///
   /// Suitable to pass to DateTime.fromMillisecondsSinceEpoch.
   DateTime get startTime;
 
   // A list of isolates running in the VM.
   Iterable<IsolateRef> get isolates;
+
+  /// Enable the sampling profiler.
+  Future enableProfiler();
 }