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

corelib/src/expect.dart

Index: corelib/src/expect.dart
diff --git a/corelib/src/expect.dart b/corelib/src/expect.dart
index c7a202793cdd4a0d63fcabaf20a3d71870cad5d1..6428ca59c5add6c1f983e3e0850aa55a03e11bd5 100644
--- a/corelib/src/expect.dart
+++ b/corelib/src/expect.dart
@@ -177,7 +177,7 @@ class Expect {
     }
     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');
   }