Fix tests in language suite: eliminate String +

tests/language/call_through_null_getter_test.dart

Index: tests/language/call_through_null_getter_test.dart
===================================================================
--- tests/language/call_through_null_getter_test.dart	(revision 7724)
+++ tests/language/call_through_null_getter_test.dart	(working copy)
@@ -52,7 +52,7 @@
     var exception = catchException(fn);
     if (!(exception is NullPointerException)) {
       Expect.fail("Wrong exception.  Expected: NullPointerException"
-          + " got: ${exception}");
+          " got: ${exception}");
     }
   }
 
@@ -60,7 +60,7 @@
     var exception = catchException(fn);
     if (!(exception is ObjectNotClosureException)) {
       Expect.fail("Wrong exception.  Expected: ObjectNotClosureException"
-          + " got: ${exception}");
+          " got: ${exception}");
     }
   }