Migrate to interpolation instead of concatenation.

lib/unittest/unittest.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A library for writing dart unit tests.
  *
  * ##Concepts##
  *
  *  * Tests: Tests are specified via the top-level function [test], they can be
@@ skipping 247 matching lines @@
 }
 
 /** Called by subclasses to indicate that an asynchronous test completed. */
 void callbackDone() {
   _callbacksCalled++;
   final testCase = _tests[_currentTest];
   if (_callbacksCalled > testCase.callbacks) {
     final expected = testCase.callbacks;
     testCase.error(
         'More calls to callbackDone() than expected. '
-        + 'Actual: ${_callbacksCalled}, expected: ${expected}', '');
+        'Actual: ${_callbacksCalled}, expected: ${expected}', '');
     _state = _UNCAUGHT_ERROR;
   } else if ((_callbacksCalled == testCase.callbacks) &&
       (_state != _RUNNING_TEST)) {
     if (testCase.result == null) testCase.pass();
     _currentTest++;
     _testRunner();
   }
 }
 
 void notifyError(String msg, String trace) {
@@ skipping 125 matching lines @@
   }
   _config.onInit();
 
   // Immediately queue the suite up. It will run after a timeout (i.e. after
   // main() has returned).
   _defer(_runTests);
 }
 
 /** Signature for a test function. */
 typedef void TestFunction();