Implement constant maps with __proto__ and fix maps with duplicated keys.

tests/language/src/CompileTimeConstantKTest.dart

Index: tests/language/src/CompileTimeConstantKTest.dart
diff --git a/tests/language/src/CompileTimeConstantKTest.dart b/tests/language/src/CompileTimeConstantKTest.dart
new file mode 100644
index 0000000000000000000000000000000000000000..7b1857ede57cbeec4c850da84ab8bfc9a3a6c559
--- /dev/null
+++ b/tests/language/src/CompileTimeConstantKTest.dart
@@ -0,0 +1,20 @@
+// 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.
+
+final x = const { 'a': 3, 'a': 4 };
+final y = const { 'a': 10, 'b': 11, 'a': 12, 'b': 13, 'a': 14 };
+final z = const { '__proto__': 496,
+                  '__proto__': 497,
+                  '__proto__': 498,
+                  '__proto__': 499 };
+
+final x2 = const { 'a': 4 };
+final y2 = const { 'a': 14, 'b': 13 };
+final z2 = const { '__proto__': 499 };
+
+main() {
+  Expect.identical(x2, x);
+  Expect.identical(y2, y);
+  Expect.identical(z2, z);
+}