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

pkg/testing/lib/src/chain.dart

 // Copyright (c) 2016, 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.md file.
 
 library testing.chain;
 
 import 'dart:async' show Future, Stream;
 
 import 'dart:convert' show JSON, JsonEncoder;
 
@@ skipping 163 matching lines @@
           future = runGuarded(() async {
             try {
               return await step.run(input, this);
             } catch (error, trace) {
               return step.unhandledError(error, trace);
             }
           }, printLineOnStdout: sb.writeln);
         } 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);
             result = currentResult;
             if (currentResult.outcome == Expectation.Pass) {
               // The input to the next step is the output of this step.
               return doStep(result.output);
             }
           }
           if (description.multitestExpectations != null) {
@@ skipping 147 matching lines @@
 /// This is called from generated code.
 Future<Null> runChain(CreateContext f, Map<String, String> environment,
     Set<String> selectors, String json) {
   return withErrorHandling(() async {
     Chain suite = new Suite.fromJsonMap(Uri.base, JSON.decode(json));
     print("Running ${suite.name}");
     ChainContext context = await f(suite, environment);
     return context.run(suite, selectors);
   });
 }