Re-land beginning Dart 2.0 test migration.

tests/corelib_2/symbol_map_helper.dart

+// Copyright (c) 2012, 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.test.symbol_map_helper;
+
+// TODO(ahe): Update map literals to avoid this method.
+Map<Symbol, dynamic> symbolMapToStringMap(Map<String, dynamic> map) {
+  if (map == null) return null;
+  Map<Symbol, dynamic> result = new Map<Symbol, dynamic>();
+  map.forEach((name, value) {
+    result[new Symbol(name)] = value;
+  });
+  return result;
+}