add string-equals to unittest, fix implementation in expect.dart

corelib/src/expect.dart

 // Copyright (c) 2011, 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.
 
 class Expect {
 
   /**
    * Checks whether the expected and actual values are equal (using [:==:]).
    */
   static void equals(var expected, var actual, [String reason = null]) {
@@ skipping 159 matching lines @@
         _fail('$defaultMessage\nDiff:\n[  ]...\n[ $snippet ]...');
         return;
       }
       if (expected[eLen - right - 1] != actual[aLen - right - 1]) {
         break;
       }
       right++;
     }
     String eSnippet = expected.substring(left, eLen - right);
     String aSnippet = actual.substring(left, aLen - right);
-    String diff = '\nDiff:\n...[ $eSnippet} ]...\n...[ $aSnippet ]...';
+    String diff = '\nDiff:\n...[ $eSnippet ]...\n...[ $aSnippet ]...';
     _fail('$defaultMessage$diff');
   }
 
   /**
    * Checks that every element of [expected] is also in [actual], and that
    * every element of [actual] is also in [expected].
    */
   static void setEquals(Iterable expected,
                         Iterable actual,
                         [String reason = null]) {
@@ skipping 60 matching lines @@
   }
 }
 
 typedef bool _CheckExceptionFn(exception);
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }