More String + elimination in tests

tests/standalone/io/read_into_const_list_test.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.
 
 // Regression test for missing immutability check in the ListSet
 // methods in the API. This allowed overwriting const Lists.
 
 #import("dart:io");
 
 String getFilename(String path) =>
-    new File(path).existsSync() ? path : 'runtime/' + path;
+    new File(path).existsSync() ? path : 'runtime/$path';
 
 void main() {
   var a = const [0];
   var b = const [0];
   Expect.isTrue(a === b);
 
   String filename = getFilename("bin/file_test.cc");
   File file = new File(filename);
   InputStream input = file.openInputStream();
   try {
     input.readInto(a, 0, 1);
     Expect.fail("no exception thrown");
   } catch (var e) {
     Expect.isTrue(e is UnsupportedOperationException);
   }
   Expect.equals(0, a[0]);
   Expect.equals(0, b[0]);
 }