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

runtime/observatory/lib/src/elements/cpu_profile/virtual_tree.dart

 // Copyright (c) 2013, 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:async';
 import 'dart:html';
 import 'dart:math' as Math;
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/src/elements/stack_trace_tree_config.dart'
     show ProfileTreeMode;
@@ skipping 110 matching lines @@
       tree = filters.fold(tree, (tree, filter) {
         return tree?.filtered(filter);
       });
     }
     if (tree == null) {
       children = [new HeadingElement.h1()..text = 'No Results'];
       return;
     }
     _tree = new VirtualTreeElement(create, update, _getChildren,
         items: tree.root.children, queue: _r.queue);
-    if (tree.root.children.length == 1) {
+    if (tree.root.children.length == 0) {
+      children = [
+        new DivElement()
+          ..classes = ['tree-item']
+          ..children = [new HeadingElement.h1()..text = 'No Samples']
+      ];
+      return;
+    } else if (tree.root.children.length == 1) {
       _tree.expand(tree.root.children.first, autoExpandSingleChildNodes: true);
     }
     children = [_tree];
   }
 
   static Element _createCpuRow(toggle) {
     return new DivElement()
       ..classes = ['tree-item']
       ..children = [
         new SpanElement()
@@ skipping 128 matching lines @@
   static _updateLines(List<Element> lines, int n) {
     n = Math.max(0, n);
     while (lines.length > n) {
       lines.removeLast();
     }
     while (lines.length < n) {
       lines.add(new SpanElement());
     }
   }
 }