Use URIs rather than paths in front end API.

pkg/front_end/lib/src/base/processed_options.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:front_end/compiler_options.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/uri_resolver.dart';
 import 'package:package_config/packages_file.dart' as package_config;
@@ skipping 21 matching lines @@
   /// Initializes a [ProcessedOptions] object wrapping the given [rawOptions].
   ProcessedOptions(CompilerOptions rawOptions) : this._raw = rawOptions;
 
   /// Determine whether to generate code for the SDK when compiling a
   /// whole-program.
   bool get compileSdk => _raw.compileSdk;
 
   /// Get the [FileSystem] which should be used by the front end to access
   /// files.
   ///
-  /// If the client supplied bazel roots using [CompilerOptions.bazelRoots], the
+  /// If the client supplied roots using [CompilerOptions.multiRoots], the
   /// returned [FileSystem] will automatically perform the appropriate mapping.
   FileSystem get fileSystem {
-    // TODO(paulberry): support bazelRoots.
-    assert(_raw.bazelRoots.isEmpty);
+    // TODO(paulberry): support multiRoots.
+    assert(_raw.multiRoots.isEmpty);
     return _raw.fileSystem;
   }
 
   /// Get the [UriResolver] which resolves "package:" and "dart:" URIs.
   ///
   /// This is an asynchronous getter since file system operations may be
   /// required to locate/read the packages file as well as SDK metadata.
   Future<UriResolver> getUriResolver() async {
     if (_uriResolver == null) {
       await _getPackages();
       var sdkLibraries =
           <String, Uri>{}; // TODO(paulberry): support SDK libraries
       _uriResolver =
           new UriResolver(_packages, sdkLibraries, fileSystem.context);
     }
     return _uriResolver;
   }
 
   /// Get the package map which maps package names to URIs.
   ///
   /// This is an asynchronous getter since file system operations may be
   /// required to locate/read the packages file.
   Future<Map<String, Uri>> _getPackages() async {
     if (_packages == null) {
       if (_raw.packagesFilePath == null) {
         throw new UnimplementedError(); // TODO(paulberry): search for .packages
-      } else if (_raw.packagesFilePath.isEmpty) {
+      } else if (_raw.packagesFilePath.path.isEmpty) {
         _packages = {};
       } else {
         var contents =
-            await fileSystem.entityForPath(_raw.packagesFilePath).readAsBytes();
-        var baseLocation = fileSystem.context.toUri(_raw.packagesFilePath);
-        _packages = package_config.parse(contents, baseLocation);
+            await fileSystem.entityForUri(_raw.packagesFilePath).readAsBytes();
+        _packages = package_config.parse(contents, _raw.packagesFilePath);
       }
     }
     return _packages;
   }
 }