Check that const-map keys don't override equals.

tests/language/const_map2_test.dart

+// Copyright (c) 2014, 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.
+
+import "package:expect/expect.dart";
+
+class A {
+  const factory A() = B;
+}
+
+class B implements A {
+  const B();
+
+  operator ==(o) => true;  /// 00: compile-time error
+}
+
+confuse(x) {
+  if (new DateTime.now() == 42) return confuse(2);
+  return x;
+}
+
+main() {
+  // It is a compile-time error if the key type overrides operator ==.
+  var m = const { const A(): 42 };
+  Expect.equals(42, m[confuse(const B())]);
+
+  m = const { "foo": 99, const A(): 499 };
+  Expect.equals(499, m[confuse(const B())]);
+}