Extract code from dependency_grapher.dart for easier re-use elsewhere in the front end.

pkg/front_end/lib/src/dependency_grapher_impl.dart

Index: pkg/front_end/lib/src/dependency_grapher_impl.dart
diff --git a/pkg/front_end/lib/dependency_grapher.dart b/pkg/front_end/lib/src/dependency_grapher_impl.dart
similarity index 71%
copy from pkg/front_end/lib/dependency_grapher.dart
copy to pkg/front_end/lib/src/dependency_grapher_impl.dart
index fcb9a210b426230a863a09de04bfae51cf32c00c..5dbe12d998d7f96c0bdbb839d03b8abeab5d07e7 100644
--- a/pkg/front_end/lib/dependency_grapher.dart
+++ b/pkg/front_end/lib/src/dependency_grapher_impl.dart
@@ -1,4 +1,4 @@
-// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2017, 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.
 
@@ -8,68 +8,30 @@ import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
 import 'package:analyzer/src/generated/parser.dart';
+import 'package:front_end/dependency_grapher.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/async_dependency_walker.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:front_end/src/base/uri_resolver.dart';
 import 'package:front_end/src/scanner/scanner.dart';
 
-import 'compiler_options.dart';
-
 /// Generates a representation of the dependency graph of a program.
 ///
 /// Given the Uri of one or more files, this function follows `import`,
 /// `export`, and `part` declarations to discover a graph of all files involved
 /// in the program.
+///
+/// This is intended for internal use by the front end.  Clients should use
+/// package:front_end/dependency_grapher.dart.
 Future<Graph> graphForProgram(
-    List<Uri> sources, CompilerOptions options) async {
-  var processedOptions = new ProcessedOptions(options);
-  var uriResolver = await processedOptions.getUriResolver();
-  var walker = new _Walker(processedOptions.fileSystem, uriResolver, processedOptions.compileSdk);
+    List<Uri> sources, ProcessedOptions options) async {
+  var uriResolver = await options.getUriResolver();
+  var walker = new _Walker(options.fileSystem, uriResolver, options.compileSdk);
   var startingPoint = new _StartingPoint(walker, sources);
   await walker.walk(startingPoint);
   return walker.graph;
 }
 
-/// A representation of the dependency graph of a program.
-///
-/// Not intended to be extended, implemented, or mixed in by clients.
-class Graph {
-  /// A list of all library cycles in the program, in topologically sorted order
-  /// (each cycle only depends on libraries in the cycles that precede it).
-  final topologicallySortedCycles = <LibraryCycleNode>[];
-
-  Graph._();
-}
-
-/// A representation of a single library cycle in the dependency graph of a
-/// program.
-///
-/// Not intended to be extended, implemented, or mixed in by clients.
-class LibraryCycleNode {
-  /// A map of all the libraries in the cycle, keyed by the URI of their
-  /// defining compilation unit.
-  final libraries = <Uri, LibraryNode>{};
-
-  LibraryCycleNode._();
-}
-
-/// A representation of a single library in the dependency graph of a program.
-///
-/// Not intended to be extended, implemented, or mixed in by clients.
-class LibraryNode {
-  /// The URI of this library's defining compilation unit.
-  final Uri uri;
-
-  /// A list of the URIs of all of this library's "part" files.
-  final parts = <Uri>[];
-
-  /// A list of all the other libraries this library directly depends on.
-  final dependencies = <LibraryNode>[];
-
-  LibraryNode._(this.uri);
-}
-
 class _Scanner extends Scanner {
   _Scanner(String contents) : super(new CharSequenceReader(contents)) {
     preserveComments = false;
@@ -95,7 +57,7 @@ class _Walker extends AsyncDependencyWalker<_WalkerNode> {
   final FileSystem fileSystem;
   final UriResolver uriResolver;
   final _nodesByUri = <Uri, _WalkerNode>{};
-  final graph = new Graph._();
+  final graph = new Graph();
   final bool compileSdk;
 
   _Walker(this.fileSystem, this.uriResolver, this.compileSdk);
@@ -108,7 +70,7 @@ class _Walker extends AsyncDependencyWalker<_WalkerNode> {
 
   @override
   Future<Null> evaluateScc(List<_WalkerNode> scc) {
-    var cycle = new LibraryCycleNode._();
+    var cycle = new LibraryCycleNode();
     for (var walkerNode in scc) {
       cycle.libraries[walkerNode.uri] = walkerNode.library;
     }
@@ -131,7 +93,7 @@ class _WalkerNode extends Node<_WalkerNode> {
 
   _WalkerNode(this.walker, Uri uri)
       : uri = uri,
-        library = new LibraryNode._(uri);
+        library = new LibraryNode(uri);
 
   @override
   Future<List<_WalkerNode>> computeDependencies() async {