Change output of string equality match failures.

pkg/unittest/test/matchers_test.dart

Index: pkg/unittest/test/matchers_test.dart
===================================================================
--- pkg/unittest/test/matchers_test.dart	(revision 23039)
+++ pkg/unittest/test/matchers_test.dart	(working copy)
@@ -506,7 +506,8 @@
       shouldPass(a, containsPair('foo', 'bar'));
       shouldFail(a, containsPair('foo', 'ba'),
           "Expected: contains pair 'foo' => 'ba' "
-          "But:  contains key 'foo' but with value was 'bar'. "
+          "But: Strings are not equal. Both strings start the same, "
+          "but the given value also has the following trailing characters: r. "
           "Actual: {'foo': 'bar'}");
       shouldFail(a, containsPair('fo', 'bar'),
           "Expected: contains pair 'fo' => 'bar' "
@@ -598,7 +599,8 @@
       var completer = new Completer();
       completer.completeError('X');
       shouldFail(completer.future, throwsA(equals('Y')),
-          "Expected: 'Y' But: was 'X'.",
+          "Expected: 'Y' But: Strings are not equal. "
+          "Expected: Y Actual: X ^ Differ at position 0.",
           isAsync: true);
     });
   });