Rename String.concat to operator+.

tests/standalone/debugger/debug_lib.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 used by debugger wire protocol tests (standalone VM debugging).
 
 library DartDebugger;
 
 import "dart:io";
 import "dart:utf";
@@ skipping 11 matching lines @@
 
 // Class to buffer wire protocol data from debug target and
 // break it down to individual json messages.
 class JsonBuffer {
   String buffer = null;
 
   append(String s) {
     if (buffer == null || buffer.length == 0) {
       buffer = s;
     } else {
-      buffer = buffer.concat(s);
+      buffer += s;
     }
   }
 
   String getNextMessage() {
     if (buffer == null) return null;
     int msgLen = objectLength();
     if (msgLen == 0) return null;
     String msg = null;
     if (msgLen == buffer.length) {
       msg = buffer;
@@ skipping 489 matching lines @@
     process.exitCode.then((int exitCode) {
       Expect.equals(0, exitCode);
       Expect.equals(0, exitCode);
       print("Debug target process exited with exit code $exitCode");
     });
     var debugger = new Debugger(process, debugPort);
     debugger.runScript(script);
   });
   return true;
 }