Eliminate uses of fuzzy arrows in pkg/testing (fix bots).

pkg/testing/lib/src/chain.dart

Index: pkg/testing/lib/src/chain.dart
diff --git a/pkg/testing/lib/src/chain.dart b/pkg/testing/lib/src/chain.dart
index 022db73b84662e1eb2e45bab0708e76c15fa0e2b..70fe9bd32ccf798345ad126993f96f15833d4a61 100644
--- a/pkg/testing/lib/src/chain.dart
+++ b/pkg/testing/lib/src/chain.dart
@@ -181,7 +181,8 @@ abstract class ChainContext {
         } else {
           future = new Future.value(null);
         }
-        future = future.then((Result currentResult) {
+        future = future.then((_currentResult) {
+          Result currentResult = _currentResult;
           if (currentResult != null) {
             logStepComplete(completed, unexpectedResults.length,
                 descriptions.length, suite, description, lastStepRun);