Use URIs rather than paths in front end API.

pkg/front_end/lib/src/dependency_grapher_impl.dart

 // 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.
 
 import 'dart:async';
 
 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';
@@ skipping 87 matching lines @@
   @override
   Future<List<_WalkerNode>> computeDependencies() async {
     var dependencies = <_WalkerNode>[];
     // TODO(paulberry): add error recovery if the file can't be read.
     var path = walker.uriResolver.resolve(uri);
     if (path == null) {
       // TODO(paulberry): If an error reporter was provided, report the error
       // in the proper way and continue.
       throw new StateError('Invalid URI: $uri');
     }
-    var contents = await walker.fileSystem.entityForPath(path).readAsString();
+    var contents = await walker.fileSystem.entityForUri(path).readAsString();

[danrubel, 2017/01/08 17:07:38]

rename "path" to "resolvedUri" ? "fileUri"?

     var scanner = new _Scanner(contents);
     var token = scanner.tokenize();
     // TODO(paulberry): report errors.
     var parser = new Parser(null, AnalysisErrorListener.NULL_LISTENER);
     var unit = parser.parseDirectives(token);
     bool coreUriFound = false;
     void handleDependency(Uri referencedUri) {
       _WalkerNode dependencyNode = walker.nodeForUri(referencedUri);
       library.dependencies.add(dependencyNode.library);
       if (referencedUri.scheme != 'dart' || walker.compileSdk) {
@@ skipping 15 matching lines @@
           handleDependency(referencedUri);
         }
       }
     }
     if (!coreUriFound) {
       handleDependency(dartCoreUri);
     }
     return dependencies;
   }
 }