Fix more strong mode errors in analyzer

pkg/analyzer/benchmark/errors_in_all_libraries.dart

 #!/usr/bin/env 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 file.
 
 /// Resolves this library and everything it transitively imports and generates
 /// errors in all of those libraries. Does this in an infinite loop, starting
 /// from scratch each time, to show how VM warm-up affects things and to make
 /// it easier to connect to this with observatory.
 import 'dart:io';
@@ skipping 22 matching lines @@
     var start = new DateTime.now();
     AnalysisEngine.instance.clearCaches();
 
     var context = AnalysisEngine.instance.createAnalysisContext();
     context.sourceFactory = new SourceFactory([
       new DartUriResolver(DirectoryBasedDartSdk.defaultSdk),
       new FileUriResolver(),
       new PackageUriResolver([new JavaFile(packageRoot)])
     ]);
 
-    context.analysisOptions.strongMode = true;
-    context.analysisOptions.strongModeHints = true;
+    AnalysisOptionsImpl options = context.analysisOptions;
+    options.strongMode = true;
+    options.strongModeHints = true;
 
     var mainSource =
         new FileBasedSource(new JavaFile(p.fromUri(Platform.script)));
     context.applyChanges(new ChangeSet()..addedSource(mainSource));
 
     var initialLibrary =
         context.resolveCompilationUnit2(mainSource, mainSource);
 
     // Walk all of the transitively referenced libraries and compute errors.
     var errorCount = 0;
@@ skipping 24 matching lines @@
   void find(LibraryElement lib) {
     if (seen.contains(lib)) return;
     seen.add(lib);
     results.add(lib);
     lib.importedLibraries.forEach(find);
     lib.exportedLibraries.forEach(find);
   }
   find(start);
   return results;
 }