Sped up hashCode by removing megamorphic call to _identityHashCode.

runtime/lib/null_patch.dart

Index: runtime/lib/null_patch.dart
diff --git a/runtime/lib/null_patch.dart b/runtime/lib/null_patch.dart
index 61972d186647457abccdffbbc5289af6d5cdb924..047039bd0f0c9ffb4e96b19e59acf9228382a847 100644
--- a/runtime/lib/null_patch.dart
+++ b/runtime/lib/null_patch.dart
@@ -11,10 +11,8 @@ patch class Null {
   }
 
   static const _HASH_CODE = 2011; // The year Dart was announced and a prime.
-  int get _identityHashCode => _HASH_CODE;
   int get hashCode => _HASH_CODE;
+  int get _identityHashCode => _HASH_CODE;
 
-  String toString() {
-    return 'null';
-  }
+  String toString() => 'null';
 }