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

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
diff --git a/runtime/lib/mirrors_impl.dart b/runtime/lib/mirrors_impl.dart
index e20aa8874d603d414c29eaca193a54e8a4bcd9d7..858b2eef12ed0be2d6b87b3f3156476ac63068f8 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -302,21 +302,11 @@ class _LocalInstanceMirrorImpl extends _LocalObjectMirrorImpl
   }
 
   int get hashCode {
-    // If the reflectee is a double or bignum, use the base hashCode to preserve
-    // the illusion that boxed numbers with the same value are identical. If the
-    // reflectee is a Smi, use the base hashCode because Object.hashCode does
-    // not work for non-heap objects. Otherwise, use Object.hashCode to maintain
-    // correctness even if a user-defined hashCode returns different values for
-    // successive invocations.
-    var h = _reflectee is num ? _reflectee.hashCode : _identityHash(_reflectee);
     // Avoid hash collisions with the reflectee. This constant is in Smi range
     // and happens to be the inner padding from RFC 2104.
-    return h ^ 0x36363636;
+    return identityHashCode(_reflectee) ^ 0x36363636;
   }
 
-  static _identityHash(reflectee)
-      native "InstanceMirror_identityHash";
-
   // Override to include the receiver in the arguments.
   InstanceMirror invoke(Symbol memberName,
                         List positionalArguments,