Add analysis option that will be used to fix #26583

pkg/analyzer/test/src/task/strong/strong_test_helper.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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 library analyzer.test.src.task.strong.strong_test_helper;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 36 matching lines @@
   files.newFile(name, content);
 }
 
 /// Run the checker on a program, staring from '/main.dart', and verifies that
 /// errors/warnings/hints match the expected value.
 ///
 /// See [addFile] for more information about how to encode expectations in
 /// the file text.
 ///
 /// Returns the main resolved library. This can be used for further checks.
-CompilationUnit check() {
+CompilationUnit check({bool implicitCasts: true}) {
   _checkCalled = true;
 
   expect(files.getFile('/main.dart').exists, true,
       reason: '`/main.dart` is missing');
 
   var uriResolver = new _TestUriResolver(files);
   // Enable task model strong mode
   var context = AnalysisEngine.instance.createAnalysisContext();
   AnalysisOptionsImpl options = context.analysisOptions as AnalysisOptionsImpl;
   options.strongMode = true;
   options.strongModeHints = true;
+  options.implicitCasts = implicitCasts;
   var mockSdk = new MockSdk();
   mockSdk.context.analysisOptions.strongMode = true;
   context.sourceFactory =
       new SourceFactory([new DartUriResolver(mockSdk), uriResolver]);
 
   // Run the checker on /main.dart.
   Source mainSource = uriResolver.resolveAbsolute(new Uri.file('/main.dart'));
   var initialLibrary = context.resolveCompilationUnit2(mainSource, mainSource);
 
   var collector = new _ErrorCollector();
@@ skipping 314 matching lines @@
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (uri.scheme == 'package') {
       return (provider.getResource('/packages/' + uri.path) as File)
           .createSource(uri);
     }
     return super.resolveAbsolute(uri, actualUri);
   }
 }