Fix tests in language suite: eliminate String +

runtime/bin/builtin.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.
 
 #library("builtin");
 #import("dart:uri");
 
 void print(arg) {
   _Logger._printString(arg.toString());
 }
@@ skipping 128 matching lines @@
   _logResolution("# Path: ${uri.path}");
   return uri.path;
 }
 
 String _filePathFromPackageUri(Uri uri) {
   if (uri.domain != '') {
     var path = (uri.path != '') ? '${uri.domain}${uri.path}' : uri.domain;
     var right = 'package:$path';
     var wrong = 'package://$path';
 
-    throw "URIs using the 'package:' scheme should look like " +
+    throw "URIs using the 'package:' scheme should look like "
           "'$right', not '$wrong'.";
   }
 
   var path;
   if (_packageRoot !== null) {
     path = "${_packageRoot}${uri.path}";
   } else {
     path = _entrypoint.resolve('packages/${uri.path}').path;
   }
 
   _logResolution("# Package: $path");
   return path;
 }