Give useful message for null derefs

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 312804e6b84c9dbb8e47654d25fc6f2ccbf62dd3..b3b9f76f4358fa60d4099d4b0b831374ba2aca65 100644
--- a/runtime/lib/null_patch.dart
+++ b/runtime/lib/null_patch.dart
@@ -14,5 +14,12 @@
   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';
 }