Fix analyzer error

pkg/dev_compiler/lib/src/analyzer/context.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.
 
 import 'package:args/args.dart' show ArgParser, ArgResults;
 import 'package:analyzer/file_system/file_system.dart'
     show ResourceProvider, ResourceUriResolver;
 import 'package:analyzer/file_system/physical_file_system.dart'
     show PhysicalResourceProvider;
 import 'package:analyzer/source/custom_resolver.dart';
@@ skipping 33 matching lines @@
 
   /// Path to the dart-sdk summary.  If this is set, it will be used in favor
   /// of the unsummarized one.
   final String dartSdkSummaryPath;
 
   AnalyzerOptions(
       {this.summaryPaths: const [],
       String dartSdkPath,
       this.dartSdkSummaryPath,
       this.customUrlMappings: const {},
+      this.packageRoot: null,

[Jennifer Messerly, 2016/09/28 17:49:05]

style nit: ": null" part is not needed.

you do need "this.packageRoot," though :)

       this.packagePaths: const []})
       : dartSdkPath = dartSdkPath ?? getSdkDir().path;
 
   AnalyzerOptions.fromArguments(ArgResults args)
       : summaryPaths = args['summary'] as List<String>,
         dartSdkPath = args['dart-sdk'] ?? getSdkDir().path,
         dartSdkSummaryPath = args['dart-sdk-summary'],
         customUrlMappings = _parseUrlMappings(args['url-mapping']),
         packageRoot = args['package-root'],
         packagePaths = (args['package-paths'] as String)?.split(',') ?? [];
@@ skipping 99 matching lines @@
   return sdk;
 }
 
 /// Creates a [DartUriResolver] that uses the SDK at the given [sdkPath].
 DartUriResolver createSdkPathResolver(String sdkSummaryPath, String sdkPath) {
   var sdk = (sdkSummaryPath != null)
       ? new SummaryBasedDartSdk(sdkSummaryPath, true)
       : _createFolderBasedDartSdk(sdkPath);
   return new DartUriResolver(sdk);
 }