Give useful message for null derefs

runtime/lib/null_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Dart core library.
 
 @patch class Null {
 
   factory Null._uninstantiable() {
     throw new UnsupportedError("class Null cannot be instantiated");
   }
 
   static const _HASH_CODE = 2011; // The year Dart was announced and a prime.
   int get hashCode => _HASH_CODE;
   int get _identityHashCode => _HASH_CODE;
 
+  @patch noSuchMethod(Invocation invocation) {
+    String name = internal.Symbol.getName(invocation.memberName);
+    if (invocation.isMethod)
+      throw new NullDereferenceError('$name() was called on a null value.');

[rmacnak, 2016/10/10 21:29:38]

DBC: "a null value" => "null"
There is only one.

+    throw new NullDereferenceError('$name was accessed on a null value.');
+  }
+
   String toString() => 'null';
 }