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

runtime/observatory/lib/src/elements/class_allocation_profile.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.
 
 import 'dart:html';
 import 'dart:async';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/src/elements/cpu_profile/virtual_tree.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
@@ skipping 55 matching lines @@
     children = [];
     _r.disable(notify: true);
   }
 
   void render() {
     if (_progress == null) {
       children = const [];
       return;
     }
     final content = [
-      new SampleBufferControlElement(_progress, _progressStream,
+      new SampleBufferControlElement(isolate.vm, _progress, _progressStream,
           selectedTag: _tag, queue: _r.queue)
         ..onTagChange.listen((e) {
           _tag = e.element.selectedTag;
-          _request();
+          _request(forceFetch: true);
         })
     ];
     if (_progress.status == M.SampleProfileLoadingStatus.loaded) {
       CpuProfileVirtualTreeElement tree;
       content.addAll([
         new BRElement(),
         new StackTraceTreeConfigElement(
             mode: _mode,
             direction: _direction,
             showFilter: false,
@@ skipping 17 matching lines @@
     _progressStream = _profiles.get(_isolate, _cls, _tag);
     _r.dirty();
     _progress = (await _progressStream.first).progress;
     _r.dirty();
     if (M.isSampleProcessRunning(_progress.status)) {
       _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
   }
 }