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

sdk/lib/core/identical.dart

Index: sdk/lib/core/identical.dart
diff --git a/sdk/lib/core/identical.dart b/sdk/lib/core/identical.dart
index 236ce20ea590f6fdcaa0c8869ff5fca850609473..cc00c30cb864f87a7d7ad2574e090d1006ed995c 100644
--- a/sdk/lib/core/identical.dart
+++ b/sdk/lib/core/identical.dart
@@ -8,3 +8,14 @@ part of dart.core;
  * Check whether two references are to the same object.
  */
 external bool identical(Object a, Object b);
+
+/**
+ * Returns the identity hash code of `object`.
+ *
+ * Returns the same value as `object.hashCode` if [object] has not overridden
+ * [Object.hashCode]. Returns the value that [Object.hashCode] would return
+ * on this object, even if `hashCode` has been overridden.
+ *
+ * This hash code is compatible with [identical].
+ */
+external int identityHashCode(Object object);