feature: tag tests; choose tags on command line

lib/src/frontend/tags.dart

Index: lib/src/frontend/tags.dart
diff --git a/lib/src/frontend/tags.dart b/lib/src/frontend/tags.dart
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+// Copyright (c) 2015, 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 test.frontend.tags;
+
+/// An annotation for applying a set of tags for a test suite.
+class Tags {
+  /// Tags applied to a test suite.
+  final tags;
+
+  /// Applies a set of tags to a test suite.
+  ///
+  /// [tags] is either an [Iterable] specifying one or more tags, or a [String]
+  /// specifying one tag.
+  const Tags([this.tags]);
+}