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

runtime/lib/object_patch.dart

Index: runtime/lib/object_patch.dart
diff --git a/runtime/lib/object_patch.dart b/runtime/lib/object_patch.dart
index 2d42da7df44ebf90db1c20f3e6b0045f3bf5533a..df8073ee4742b872b6bdc15a3385641e740051af 100644
--- a/runtime/lib/object_patch.dart
+++ b/runtime/lib/object_patch.dart
@@ -12,15 +12,27 @@ patch class Object {
   static _getHash(obj) native "Object_getHash";
   static _setHash(obj, hash) native "Object_setHash";
 
-  /* patch */ int get hashCode {
-    var result = _getHash(this);
+  /* patch */ int get hashCode => _hashCodeOf(this);
+
+  /* patch */ static int hashCodeOf(Object object) {
+    // Num doesn't use identity hash for hashCode, and this method must be
+    // consistent with `identical`.
+    // Bool and Null overrides hashCode only in a patch.
+    if (object is num || object is bool || object == null) {
+      return object.hashCode;
+    }
+    return _hashCodeOf(object);
+  }
+
+  static int _hashCodeOf(Object object) {
+    var result = _getHash(object);
     if (result == 0) {
       // We want the hash to be a Smi value greater than 0.
       result = _hashCodeRnd.nextInt(0x40000000);
       while (result == 0) {
         result = _hashCodeRnd.nextInt(0x40000000);
       }
-      _setHash(this, result);
+      _setHash(object, result);
     }
     return result;
   }