Update get$hashCode to not use JS for most uses. Also fix issue 6522 where the hashCode of a String…

tests/compiler/dart2js_extra/hash_code_test.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.
+
+// dart2js specific test to make sure String.hashCode behaves as
+// intended.
+
+main() {
+  Expect.equals(67633152, 'a'.hashCode);

[sra1, 2012/11/12 18:20:58]

= 0x04080000

+  Expect.equals(37224448, 'b'.hashCode);

[sra1, 2012/11/12 18:20:58]

= 0x02380000

There is something very wrong with these numbers.

[ahe, 2012/11/12 20:35:23]

I assume the problem is that these two value have roughly the same bits?

+}