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

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index aa2267efc0a9be8ad733fc28ff7e569242e77d53..cec9518d5c1c9c0caec0f9536f2499b28dbc9389 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -222,7 +222,7 @@ class _Smi extends _IntegerImplementation implements int {
     throw new UnsupportedError(
         "_Smi can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Smi_bitNegate";
@@ -271,7 +271,7 @@ class _Mint extends _IntegerImplementation implements int {
     throw new UnsupportedError(
         "_Mint can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Mint_bitNegate";
@@ -295,7 +295,7 @@ class _Bigint extends _IntegerImplementation implements int {
     throw new UnsupportedError(
         "_Bigint can only be allocated by the VM");
   }
-  int get hashCode {
+  int get _identityHashCode {
     return this;
   }
   int operator ~() native "Bigint_bitNegate";