Initial UserTag and dart:profiler library

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
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+// 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 {
+  external factory UserTag(String label);
+
+  /// Label of [this].
+  String get label;
+
+  /// Make [this] the current tag.
+  makeCurrent();
+}