Do not always error-out when evaluating a map that is not a compile time constant.

tests/language/lazy_map_test.dart

Index: tests/language/lazy_map_test.dart
===================================================================
--- tests/language/lazy_map_test.dart	(revision 0)
+++ tests/language/lazy_map_test.dart	(revision 0)
@@ -0,0 +1,8 @@
+// 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.
+
+var data = { 'a': 'a' };
+main() {
+  Expect.equals('a', data['a']);
+}