Refactor 'dart:profiler' UserTag API

sdk/lib/profiler/profiler.dart

Index: sdk/lib/profiler/profiler.dart
diff --git a/sdk/lib/profiler/profiler.dart b/sdk/lib/profiler/profiler.dart
index 56614ddec0c5cc7fb4db5bee186b9b692463b43d..ff07673335f8d06431aebd896a35af585f6d5cd9 100644
--- a/sdk/lib/profiler/profiler.dart
+++ b/sdk/lib/profiler/profiler.dart
@@ -14,8 +14,12 @@ abstract class UserTag {
   /// 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
@@ -45,22 +49,18 @@ class _FakeUserTag implements UserTag {
 
   final String label;
 
-  makeCurrent() {
+  UserTag makeCurrent() {
+    var old = _currentTag;
     _currentTag = this;
+    return _currentTag;

[Ivan Posva, 2014/05/06 16:43:06]

Shouldn't you be returning old here?

[Cutch, 2014/05/06 17:00:25]

Done.

   }
+
+  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;
-}