- Fix use of String operator + in language and corelib tests.

tests/language/src/InterfaceFactoryTest.dart

 // Copyright (c) 2012, 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.
 //
 // Test that the two major variations of interface factories work.
 
 // Variant 1.  The factory class implements the interface and provides
 // a default implementation of the interface.
 
 interface Interface1 default DefaultImplementation {
@@ skipping 24 matching lines @@
 interface Interface2 default FactoryProvider {
   Interface2(var secret);
   Interface2.named();
 
   GetSecret();
 }
 
 class SomeImplementation implements Interface2 {
   String _secret;
 
-  SomeImplementation(String one, String two) : _secret = one + two {}
+  SomeImplementation(String one, String two) : _secret = "${one}${two}" {}
 
   String GetSecret() { return _secret; }
 }
 
 // Note that FactoryProvider does not implement Interface2.
 class FactoryProvider {
   factory Interface2(var secret) {
     return new SomeImplementation(secret, secret);
   }
 
   factory Interface2.named() {
     return new SomeImplementation("Named", "Constructor");
   }
 
   static testMain() {
     Expect.equals("cobracobra", new Interface2("cobra").GetSecret());
     Expect.equals("NamedConstructor", new Interface2.named().GetSecret());
   }
 }
 
 main() {
   DefaultImplementation.testMain();
   FactoryProvider.testMain();
 }