Rename String.concat to operator+.

tools/ddbg.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.
 
 // Simple interactive debugger shell that connects to the Dart VM's debugger
 // connection port.
 
 import "dart:io";
 import "dart:json" as json;
 import "dart:utf";
@@ skipping 436 matching lines @@
     }
   }
 }
 
 
 void processVmData(String data) {
   final printMessages = false;
   if (vmData == null || vmData.length == 0) {
     vmData = data;
   } else {
-    vmData = vmData.concat(data);
+    vmData = vmData + data;
   }
   int msg_len = jsonObjectLength(vmData);
   if (printMessages && msg_len == 0) {
     print("have partial or illegal json message"
           " of ${vmData.length} chars:\n'$vmData'");
     return;
   }
   while (msg_len > 0 && msg_len <= vmData.length) {
     if (msg_len == vmData.length) {
       if (printMessages) { print("have one full message:\n$vmData"); }
@@ skipping 90 matching lines @@
       process.stdin.close();
       process.exitCode.then((int exitCode) {
         print('${arguments.join(" ")} exited with $exitCode');
       });
       debuggerMain();
     });
   } else {
     debuggerMain();
   }
 }