Fix issue 4191.

pkg/unittest/mock.dart

 // Copyright (c) 2012, 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.
 
 /**
  * The error formatter for mocking is a bit different from the default one
  * for unit testing; instead of the third argument being a 'reason'
  * it is instead a [signature] describing the method signature filter
  * that was used to select the logs that were verified.
  */
@@ skipping 92 matching lines @@
               arg6 = _noArg,
               arg7 = _noArg,
               arg8 = _noArg,
               arg9 = _noArg]) {
     if (name == null) {
       nameFilter = anything;
     } else {
       nameFilter = wrapMatcher(name);
     }
     argMatchers = new List<Matcher>();
-    if (arg0 == _noArg) return;
+    if (arg0 === _noArg) return;
     argMatchers.add(wrapMatcher(arg0));
-    if (arg1 == _noArg) return;
+    if (arg1 === _noArg) return;
     argMatchers.add(wrapMatcher(arg1));
-    if (arg2 == _noArg) return;
+    if (arg2 === _noArg) return;
     argMatchers.add(wrapMatcher(arg2));
-    if (arg3 == _noArg) return;
+    if (arg3 === _noArg) return;
     argMatchers.add(wrapMatcher(arg3));
-    if (arg4 == _noArg) return;
+    if (arg4 === _noArg) return;
     argMatchers.add(wrapMatcher(arg4));
-    if (arg5 == _noArg) return;
+    if (arg5 === _noArg) return;
     argMatchers.add(wrapMatcher(arg5));
-    if (arg6 == _noArg) return;
+    if (arg6 === _noArg) return;
     argMatchers.add(wrapMatcher(arg6));
-    if (arg7 == _noArg) return;
+    if (arg7 === _noArg) return;
     argMatchers.add(wrapMatcher(arg7));
-    if (arg8 == _noArg) return;
+    if (arg8 === _noArg) return;
     argMatchers.add(wrapMatcher(arg8));
-    if (arg9 == _noArg) return;
+    if (arg9 === _noArg) return;
     argMatchers.add(wrapMatcher(arg9));
   }
 
   /**
    * We keep our behavior specifications in a Map, which is keyed
    * by the [CallMatcher]. To make the keys unique and to get a
    * descriptive value for the [CallMatcher] we have this override
    * of [toString()].
    */
   String toString() {
@@ skipping 1308 matching lines @@
       }
     }
   }
 
   /** Clear both logs and behavior. */
   void reset() {
     resetBehavior();
     clearLogs();
   }
 }