If two Maps are compared with an equals() matcher and they have the same length,

pkg/unittest/lib/src/core_matchers.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.
 
 part of matcher;
 
 /**
  * Returns a matcher that matches empty strings, maps or iterables
  * (including collections).
  */
@@ skipping 117 matching lines @@
     Description reason = null;
     // If _limit is 1 we can only recurse one level into object.
     bool canRecurse = depth == 0 || _limit > 1;
     if (expected == actual) {
       // Do nothing.
     } else if (depth > _limit) {
       reason = new StringDescription('recursion depth limit exceeded');
     } else {
       if (expected is Iterable && canRecurse) {
         String r = _compareIterables(expected, actual,
-          _recursiveMatch, depth+1);
+            _recursiveMatch, depth+1);
         if (r != null) reason = new StringDescription(r);
       } else if (expected is Map && canRecurse) {
         if (actual is !Map) {
           reason = new StringDescription('expected a map');
-        } else if (expected.length != actual.length) {
-          reason = new StringDescription('different map lengths');
         } else {
+          var err = (expected.length == actual.length) ? '' :
+                    'different map lengths; ';
           for (var key in expected.keys) {
             if (!actual.containsKey(key)) {
-              reason = new StringDescription('missing map key ');
+              reason = new StringDescription(err);
+              reason.add('missing map key ');
               reason.addDescriptionOf(key);
               break;
             }
-            reason = _recursiveMatch(expected[key], actual[key],
-                'with key <${key}> ${location}', depth+1);
-            if (reason != null) {
-              break;
+          }
+          if (reason == null) {
+            for (var key in actual.keys) {
+              if (!expected.containsKey(key)) {
+                reason = new StringDescription(err);
+                reason.add('extra map key ');
+                reason.addDescriptionOf(key);
+                break;
+              }
+            }
+            if (reason == null) {
+              for (var key in expected.keys) {
+                reason = _recursiveMatch(expected[key], actual[key],
+                    'with key <${key}> ${location}', depth+1);
+                if (reason != null) {
+                  break;
+                }
+              }
             }
           }
         }
       } else {
         // If we have recursed, show the expected value too; if not,
         // expect() will show it for us.
         reason = new StringDescription();
         if (depth > 1) {
           reason.add('expected ').addDescriptionOf(expected).add(' but was ').
               addDescriptionOf(actual);
@@ skipping 531 matching lines @@
       description.add(_featureDescription).add(' ').addDescriptionOf(_matcher);
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     mismatchDescription.add(_featureName).add(' ');
     _matcher.describeMismatch(matchState.state['feature'], mismatchDescription,
         matchState.state['innerState'], verbose);
     return mismatchDescription;
   }
 }