Use URIs rather than paths in front end API.

pkg/front_end/lib/src/base/uri_resolver.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.
 
 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?
 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.
   final Map<String, Uri> packages;
 
   /// A map from SDK library name (e.g. `core` for `dart:core`) to the file URI
   /// 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';
-
-  /// 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 file path it
+  /// corresponds 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`.
+  ///
+  /// 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
+  /// converted to relative file paths).
   ///
   /// Note that no I/O is performed; the file path 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;
       String rest;
       if (slashIndex >= 0) {
         prefix = path.substring(0, slashIndex);
         rest = path.substring(slashIndex + 1);
       } else {
         prefix = path;
         rest = '';
       }
       Uri libUri;
       if (uri.scheme == PACKAGE_SCHEME) {
         if (slashIndex < 0) return null;
         libUri = packages[prefix];
       } else if (uri.scheme == DART_SCHEME) {
         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);
   }
 }