Use URIs rather than paths in front end API.

pkg/front_end/lib/src/base/uri_resolver.dart

Index: pkg/front_end/lib/src/base/uri_resolver.dart
diff --git a/pkg/front_end/lib/src/base/uri_resolver.dart b/pkg/front_end/lib/src/base/uri_resolver.dart
index db469f0003df510d13a843e5714d15aa220f65be..0c6a4209743afe02849f32f3ca51c2a0948c90ae 100644
--- a/pkg/front_end/lib/src/base/uri_resolver.dart
+++ b/pkg/front_end/lib/src/base/uri_resolver.dart
@@ -2,13 +2,15 @@
 // 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:path/path.dart' as p;
-
-/// The class `UriResolver` implements the rules for resolving URIs to file
-/// paths.
-///
-/// TODO(paulberry): Is it necessary to support the "http" scheme?
+/// The class `UriResolver` implements the rules for resolving "dart:" and
+/// "package:" URIs.
 class UriResolver {
+  /// The URI scheme used for "package" URIs.
+  static const PACKAGE_SCHEME = 'package';
+
+  /// The URI scheme used for "dart" URIs.
+  static const DART_SCHEME = 'dart';
+
   /// A map from package name to the file URI of the "lib" directory of the
   /// corresponding package.  This is equivalent to the format returned by
   /// the "package_config" package's parse() function.
@@ -18,36 +20,27 @@ class UriResolver {
   /// of the defining compilation unit of the SDK library.
   final Map<String, Uri> sdkLibraries;
 
-  /// The path context which should be used to convert from file URIs to file
-  /// paths.
-  final p.Context pathContext;
-
-  /// The URI scheme used for "package" URIs.
-  static const PACKAGE_SCHEME = 'package';
+  UriResolver(this.packages, this.sdkLibraries);
 
-  /// The URI scheme used for "dart" URIs.
-  static const DART_SCHEME = 'dart';
-
-  /// The URI scheme used for "file" URIs.
-  static const FILE_SCHEME = 'file';
-
-  UriResolver(this.packages, this.sdkLibraries, this.pathContext);
-
-  /// Converts a URI to a file path.
+  /// Converts "package:" and "dart:" URIs to the locations of the corresponding
+  /// files.
   ///
-  /// If the given URI is valid, and of a recognized form, returns the file path
-  /// it corresponds to.  Otherwise returns `null`.  It is not necessary for the
-  /// URI to be absolute (relative URIs will be converted to relative file
-  /// paths).
+  /// If the given URI is a "package:" or "dart:" URI, is well formed, and names
+  /// a package or dart library that is recognized, returns the URI it resolves
+  /// to.  If the given URI is a "package:" or "dart:" URI, and is ill-formed
+  /// or names a package or dart library that is not recognized, returns `null`.
   ///
-  /// Note that no I/O is performed; the file path that is returned will be
+  /// If the given URI has any scheme other than "package:" or "dart:", it is
+  /// returned unchanged.
+  ///
+  /// It is not necessary for the URI to be absolute (relative URIs will be
+  /// passed through unchanged).
+  ///
+  /// Note that no I/O is performed; the URI that is returned will be
   /// independent of whether or not any particular file exists on the file
   /// system.
-  String resolve(Uri uri) {
-    Uri fileUri;
-    if (uri.scheme == FILE_SCHEME) {
-      fileUri = uri;
-    } else {
+  Uri resolve(Uri uri) {
+    if (uri.scheme == DART_SCHEME || uri.scheme == PACKAGE_SCHEME) {
       var path = uri.path;
       var slashIndex = path.indexOf('/');
       String prefix;
@@ -67,9 +60,9 @@ class UriResolver {
         libUri = sdkLibraries[prefix];
       }
       if (libUri == null) return null;
-      fileUri = libUri.resolve(rest);
-      if (fileUri.scheme != FILE_SCHEME) return null;
+      return libUri.resolve(rest);
+    } else {
+      return uri;
     }
-    return pathContext.fromUri(fileUri);
   }
 }