Fix tests in language suite: eliminate String +

tests/language/call_through_null_getter_test.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.
 
 // Tests that we can call functions through getters which return null.
 
 final TOP_LEVEL_NULL = null;
 
 var topLevel;
 
@@ skipping 34 matching lines @@
     A a = new A();
 
     a.field = null;
     expectThrowsObjectNotClosureException(() { a.method()(); });
   }
 
   static void expectThrowsNullPointerException(fn) {
     var exception = catchException(fn);
     if (!(exception is NullPointerException)) {
       Expect.fail("Wrong exception.  Expected: NullPointerException"
-          + " got: ${exception}");
+          " got: ${exception}");
     }
   }
 
   static void expectThrowsObjectNotClosureException(fn) {
     var exception = catchException(fn);
     if (!(exception is ObjectNotClosureException)) {
       Expect.fail("Wrong exception.  Expected: ObjectNotClosureException"
-          + " got: ${exception}");
+          " got: ${exception}");
     }
   }
 
   static catchException(fn) {
     bool caught = false;
     var result = null;
     try {
       fn();
       Expect.equals(true, false);  // Shouldn't reach this.
     } catch (var e) {
@@ skipping 12 matching lines @@
   A() { }
   var field;
   get getter() { return field; }
   method() { return field; }
 
 }
 
 main() {
   CallThroughNullGetterTest.testMain();
 }