Rename String.concat to operator+.

tools/testing/dart/test_runner.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.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 1132 matching lines @@
       print('Error on batch runner input stream stdin');
       print('  Input line: $line');
       print('  Previous test\'s status: $_status');
       print('  Error: $err');
       throw err;
     };
     _process.stdin.write(line.charCodes);
   }
 
   String _createArgumentsLine(List<String> arguments) {
-    return arguments.join(' ').concat('\n');
+    return arguments.join(' ') + '\n';
   }
 
   void _reportResult() {
     if (!active) return;
     // _status == '>>> TEST {PASS, FAIL, OK, CRASH, FAIL, TIMEOUT}'
 
     var outcome = _status.split(" ")[2];
     var exitCode = 0;
     if (outcome == "CRASH") exitCode = CRASHING_BROWSER_EXITCODE;
     if (outcome == "FAIL" || outcome == "TIMEOUT") exitCode = 1;
@@ skipping 582 matching lines @@
           completer.complete(testCase);
         }
       });
     }
     runCommand();
 
     return completer.future;
   }
 }