Fix static error in test_runner_test.

tests/standalone/io/test_runner_test.dart

 // Copyright (c) 2013, 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.
 
 import "dart:io";
 import "dart:isolate";
 import "dart:async";
 import "../../../tools/testing/dart/expectation.dart";
 import "../../../tools/testing/dart/options.dart";
 import "../../../tools/testing/dart/status_file.dart";
@@ skipping 51 matching lines @@
     if (numTests != numCompletedTests) {
       throw "bad completion count. "
           "expected: $numTests, actual: $numCompletedTests";
     }
   }
 }
 
 class CustomTestSuite extends TestSuite {
   CustomTestSuite(Map configuration) : super(configuration, "CustomTestSuite");
 
-  void forEachTest(TestCaseEvent onTest, Map testCache, [onDone]) {
+  Future forEachTest(TestCaseEvent onTest, Map testCache, [onDone]) {
     void enqueueTestCase(testCase) {
       TestController.numTests++;
       onTest(testCase);
     }
 
     var testCaseCrash = _makeCrashTestCase("crash", [Expectation.crash]);
     var testCasePass = _makeNormalTestCase("pass", [Expectation.pass]);
     var testCaseFail = _makeNormalTestCase("fail", [Expectation.fail]);
     var testCaseTimeout = _makeNormalTestCase("timeout", [Expectation.timeout]);
     var testCaseFailUnexpected =
         _makeNormalTestCase("fail-unexpected", [Expectation.pass]);
 
     enqueueTestCase(testCaseCrash);
     enqueueTestCase(testCasePass);
     enqueueTestCase(testCaseFail);
     enqueueTestCase(testCaseTimeout);
     enqueueTestCase(testCaseFailUnexpected);
 
     if (onDone != null) {
       onDone();
     }
+
+    return null;

[Paul Berry, 2017/05/30 22:26:29]

It's not obvious why this is ok.  Can we add a comment saying something like
"the test framework doesn't use the return value"?

Alternatively, just return `new Future.value()`.

   }
 
   TestCase _makeNormalTestCase(name, expectations) {
     var args = packageOptions();
     args.addAll([Platform.script.toFilePath(), name]);
     var command = CommandBuilder.instance
         .getProcessCommand('custom', Platform.executable, args, {});
     return _makeTestCase(name, DEFAULT_TIMEOUT, command, expectations);
   }
 
@@ skipping 53 matching lines @@
         break;
       case 'timeout':
         // This process should be killed by the test after DEFAULT_TIMEOUT
         new Timer(new Duration(hours: 42), () {});
         break;
       default:
         throw "Unknown option ${arguments[0]} passed to test_runner_test";
     }
   }
 }