More strong mode fixes for analyzer

pkg/analyzer/test/source/error_processor_test.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.test.source.error_processor;
 
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:analyzer/source/error_processor.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 77 matching lines @@
 analyzer:
   errors:
     invalid_assignment: unsupported_action # should be skipped
     missing_return: false
     unused_local_variable: error
 ''';
 
     group('processing', () {
       test('yaml map', () {
         var options = optionsProvider.getOptionsFromString(config);
-        var errorConfig = new ErrorConfig(options['analyzer']['errors']);
+        var errorConfig =
+            new ErrorConfig((options['analyzer'] as YamlMap)['errors']);
         expect(errorConfig.processors, hasLength(2));
 
         // ignore
         var missingReturnProcessor = errorConfig.processors
             .firstWhere((p) => p.appliesTo(missing_return));
         expect(missingReturnProcessor.severity, isNull);
 
         // error
         var unusedLocalProcessor = errorConfig.processors
             .firstWhere((p) => p.appliesTo(unused_local_variable));
@@ skipping 51 matching lines @@
 void oneTimeSetup() {
   List<Plugin> plugins = <Plugin>[];
   plugins.addAll(AnalysisEngine.instance.requiredPlugins);
   plugins.add(AnalysisEngine.instance.commandLinePlugin);
   plugins.add(AnalysisEngine.instance.optionsPlugin);
   ExtensionManager manager = new ExtensionManager();
   manager.processPlugins(plugins);
 }
 
 class TestContext extends AnalysisContextImpl {}