Fix bug in partial parsing of map literal.

dart/tests/language/src/MapLiteralSyntaxTest.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.
+
+class Foo {
+  var x;
+  var y;
+  var z;
+  var v;
+  Foo() : x = {}, y = <int>{}, z = const {}, v = const <int>{};
+}
+
+main() {
+  Expect.equals("{}", new Foo().x.toString());
+  Expect.equals("{}", new Foo().y.toString());
+  Expect.equals("{}", new Foo().z.toString());
+  Expect.equals("{}", new Foo().v.toString());
+}