Refactor 'dart:profiler' UserTag API

runtime/lib/profiler.dart

Index: runtime/lib/profiler.dart
diff --git a/runtime/lib/profiler.dart b/runtime/lib/profiler.dart
index 5ed8df5cfcd063240988d5678dff4a7e6e1913a6..c42e9668a5b3a59580f44094dd9faf5ed5d0b706 100644
--- a/runtime/lib/profiler.dart
+++ b/runtime/lib/profiler.dart
@@ -8,6 +8,7 @@ patch class UserTag {
   /* patch */ factory UserTag(String label) {
     return new _UserTag(label);
   }
+  /* patch */ static UserTag get defaultTag => _getDefaultTag();
 }
 
 
@@ -20,5 +21,4 @@ class _UserTag implements UserTag {
 patch UserTag getCurrentTag() => _getCurrentTag();
 UserTag _getCurrentTag() native "Profiler_getCurrentTag";
 
-patch UserTag clearCurrentTag() => _clearCurrentTag();
-UserTag _clearCurrentTag() native "Profiler_clearCurrentTag";
+UserTag _getDefaultTag() native "UserTag_defaultTag";