Add preliminary packages file support to dependency_grapher.

pkg/front_end/lib/compiler_options.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.
 
 library front_end.compiler_options;
 
 import 'compilation_error.dart';
 import 'file_system.dart';
 import 'physical_file_system.dart';
 
@@ skipping 18 matching lines @@
   /// Callback to which compilation errors should be delivered.
   ///
   /// By default, the first error will be reported by throwing an exception of
   /// type [CompilationError].
   ErrorHandler onError = defaultErrorHandler;
 
   /// Path to the ".packages" file.
   ///
   /// If `null`, the ".packages" file will be found via the standard
   /// package_config search algorithm.
+  ///
+  /// If the empty string, no packages file will be used.
   String packagesFilePath;
 
   /// Paths to the input summary files (excluding the SDK summary).  These files
   /// should all be linked summaries.  They should also be closed, in the sense
   /// that any libraries they reference should also appear in [inputSummaries]
   /// or [sdkSummary].
   List<String> inputSummaries = [];
 
   /// Path to the SDK summary file.
   ///
@@ skipping 63 matching lines @@
   /// that will be compiled are listed explicitly and all other dependencies
   /// are covered by summary files.
   ///
   /// When this option is true, these APIs will treat any dependency that is
   /// not described in a summary as if it was explictly listed as an input.
   bool chaseDependencies = false;
 
   /// Whether to intepret Dart sources in strong-mode.
   bool strongMode = true;
 }