Make the implementations of InstanceMirror.hashCode use dart:core's identityHashCode.

tests/corelib/hashcode_boxed_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.
+
+import "package:expect/expect.dart";
+
+double fib(double n) {
+  return n <= 1.0 ? 1.0 : fib(n-1) + fib(n-2);
+}
+
+main() {
+  // Compute the same value in a way that won't be optimized away so the results
+  // are different objects in memory.
+  var a = fib(5.0) + 1.0;
+  var b = fib(4.0) + 4.0;
+
+  Expect.isTrue(identical(a, b));
+  Expect.equals(identityHashCode(a), identityHashCode(b));
+  Expect.equals(a, b);
+  Expect.equals(a.hashCode, b.hashCode);
+}