Make the VM's dart:core and dart:async library patches clean.

sdk/lib/core/null.dart

Index: sdk/lib/core/null.dart
diff --git a/sdk/lib/core/null.dart b/sdk/lib/core/null.dart
index dd914a3be90ae0ea66c2eabf9c7a0a5263f011af..37d16de865fbdffed361d1a8fac6c02c2e35bab0 100644
--- a/sdk/lib/core/null.dart
+++ b/sdk/lib/core/null.dart
@@ -16,6 +16,9 @@ class Null {
     throw new UnsupportedError('class Null cannot be instantiated');
   }
 
+  // TODO: Doc comment please.

[Lasse Reichstein Nielsen, 2016/12/08 12:25:32]

No doc needed.

[Kevin Millikin (Google), 2016/12/09 08:38:59]

Done.

+  external int get hashCode;
+
   /** Returns the string `"null"`. */
   String toString() => "null";
 }