Refactor 'dart:profiler' UserTag API

sdk/lib/profiler/profiler.dart

 // Copyright (c) 2014, 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.
 
 library dart.profiler;
 
 /// A UserTag can be used to group samples in the Observatory profiler.
 abstract class UserTag {
   /// The maximum number of UserTag instances that can be created by a program.
   static const MAX_USER_TAGS = 64;
 
   factory UserTag(String label) => new _FakeUserTag(label);
 
   /// Label of [this].
   String get label;
 
-  /// Make [this] the current tag for the isolate.
-  makeCurrent();
+  /// Make [this] the current tag for the isolate. Returns the current tag
+  /// before setting.
+  UserTag makeCurrent();
+
+  /// The default [UserTag] with label 'Default'.
+  static UserTag get defaultTag => _FakeUserTag._defaultTag;
 }
 
 // This is a fake implementation of UserTag so that code can compile and run
 // in dart2js.
 class _FakeUserTag implements UserTag {
-  static List _instances = [];
+  static Map _instances = {};
 
   _FakeUserTag.real(this.label);
 
   factory _FakeUserTag(String label) {
     // Canonicalize by name.
-    for (var tag in _instances) {
-      if (tag.label == label) {
-        return tag;
-      }
+    var existingTag = _instances[label];
+    if (existingTag != null) {
+      return existingTag;
     }
     // Throw an exception if we've reached the maximum number of user tags.
     if (_instances.length == UserTag.MAX_USER_TAGS) {
       throw new UnsupportedError(
           'UserTag instance limit (${UserTag.MAX_USER_TAGS}) reached.');
     }
-    // Create a new instance and add it to the instance list.
+    // Create a new instance and add it to the instance map.
     var instance = new _FakeUserTag.real(label);
-    _instances.add(instance);
+    _instances[label] = instance;
     return instance;
   }
 
   final String label;
 
-  makeCurrent() {
+  UserTag makeCurrent() {
+    var old = _currentTag;
     _currentTag = this;
+    return old;
   }
+
+  static final UserTag _defaultTag = new _FakeUserTag('Default');
 }
 
-var _currentTag = null;
+var _currentTag = _FakeUserTag._defaultTag;
 
 /// Returns the current [UserTag] for the isolate.
 UserTag getCurrentTag() {
   return _currentTag;
 }
-
-/// Sets the current [UserTag] for the isolate to null. Returns current tag
-/// before clearing.
-UserTag clearCurrentTag() {
-  var old = _currentTag;
-  _currentTag = null;
-  return old;
-}