Fix static error in test_runner_test.

tests/standalone/io/test_runner_test.dart

Index: tests/standalone/io/test_runner_test.dart
diff --git a/tests/standalone/io/test_runner_test.dart b/tests/standalone/io/test_runner_test.dart
index 90920e93221eaf8283734543501ea4c7198eee8c..2b13cdaa2a8aac05a6f0035e08de0356109ce4da 100644
--- a/tests/standalone/io/test_runner_test.dart
+++ b/tests/standalone/io/test_runner_test.dart
@@ -69,7 +69,7 @@ class TestController {
 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);
@@ -91,6 +91,8 @@ class CustomTestSuite extends TestSuite {
     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) {