More String + elimination in tests

tests/standalone/io/file_input_stream_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.
 // Testing file input stream, VM-only, standalone test.
 
 #import("dart:io");
 #import("dart:isolate");
 
 // Helper method to be able to run the test from the runtime
 // directory, or the top directory.
 String getFilename(String path) =>
-    new File(path).existsSync() ? path : '../' + path;
+    new File(path).existsSync() ? path : '../$path';
 
 void testStringInputStreamSync() {
   String fileName = getFilename("tests/standalone/io/readuntil_test.dat");
   // File contains "Hello Dart\nwassup!\n"
   File file = new File(fileName);
   StringInputStream x = new StringInputStream(file.openInputStream());
   x.onLine = () {
     // The file input stream is known (for now) to have read the whole
     // file when the data handler is called.
     String line = x.readLine();
@@ skipping 72 matching lines @@
 main() {
   testStringInputStreamSync();
   testInputStreamAsync();
   // Check the length of these files as both are text files where one
   // is without a terminating line separator which can easily be added
   // back if accidentally opened in a text editor.
   testStringInputStreamAsync("readline_test1.dat", 111);
   testStringInputStreamAsync("readline_test2.dat", 114);
   testChunkedInputStream();
 }