Support general expressions as keys in literal maps.

tests/language/map_literal9_test.dart

+// Copyright (c) 2013, 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.
+
+// Test the use type arguments on constant maps.
+
+library map_literal9_test;
+
+import "package:expect/expect.dart";
+
+void main() {
+  var m1 = const {"[object Object]": 0, "1": 1};
+  Expect.isFalse(m1.containsKey(new Object()));
+  Expect.isNull(m1[new Object()]);
+  Expect.isFalse(m1.containsKey(1));
+  Expect.isNull(m1[1]);
+
+  var m2 = const {"[object Object]": 0, "1": 1, "__proto__": 2};
+  Expect.isFalse(m2.containsKey(new Object()));
+  Expect.isNull(m2[new Object()]);
+  Expect.isFalse(m2.containsKey(1));
+  Expect.isNull(m2[1]);
+}