Add Object.hashCodeOf to get the default hashCode of an object.

runtime/lib/integers.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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
 class _IntegerImplementation {
   factory _IntegerImplementation._uninstantiable() {
     throw new UnsupportedError(
         "_IntegerImplementation can only be allocated by the VM");
@@ skipping 204 matching lines @@
   }
 
   _leftShiftWithMask32(count, mask)  native "Integer_leftShiftWithMask32";
 }
 
 class _Smi extends _IntegerImplementation implements int {
   factory _Smi._uninstantiable() {
     throw new UnsupportedError(
         "_Smi can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Smi_bitNegate";
   int get bitLength native "Smi_bitLength";
 
   int _shrFromInt(int other) native "Smi_shrFromInt";
   int _shlFromInt(int other) native "Smi_shlFromInt";
 
   String toString() {
     if (this == 0) return "0";
@@ skipping 28 matching lines @@
 
 // Reusable buffer used by smi.toString.
 List _toStringBuffer = new Uint8List(20);
 
 // Represents integers that cannot be represented by Smi but fit into 64bits.
 class _Mint extends _IntegerImplementation implements int {
   factory _Mint._uninstantiable() {
     throw new UnsupportedError(
         "_Mint can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Mint_bitNegate";
   int get bitLength native "Mint_bitLength";
 
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }
 
 // A number that can be represented as Smi or Mint will never be represented as
 // Bigint.
 class _Bigint extends _IntegerImplementation implements int {
   factory _Bigint._uninstantiable() {
     throw new UnsupportedError(
         "_Bigint can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Bigint_bitNegate";
   int get bitLength native "Bigint_bitLength";
 
   // Shift by bigint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Bigint_shlFromInt";
 
   int pow(int exponent) {
     throw "Bigint.pow not implemented";
   }
 }