Don't do Uri->String->Uri when resolving package imports.

runtime/bin/builtin.dart

 // Copyright (c) 2012, 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.
 
 library builtin;
 import 'dart:io';
 import 'dart:async';
 import 'dart:convert';
 // import 'root_library'; happens here from C Code
 
@@ skipping 158 matching lines @@
   if (_logBuiltin) {
     _print('# Getting file path from: $uri');
   }
 
   var path;
   switch (uri.scheme) {
     case '':
     case 'file':
       return uri.toFilePath();
     case 'package':
-      return _filePathFromPackageUri(uri);
+      return _filePathFromUri(_resolvePackageUri(uri).toString());
     case 'http':
       return uri.toString();
     default:
       // Only handling file, http, and package URIs
       // in standalone binary.
       if (_logBuiltin) {
         _print('# Unknown scheme (${uri.scheme}) in $uri.');
       }
       throw 'Not a known scheme: $uri';
   }
 }
 
 
-String _filePathFromPackageUri(Uri uri) {
+Uri _resolvePackageUri(Uri uri) {
   if (!uri.host.isEmpty) {
     var path = '${uri.host}${uri.path}';
     var right = 'package:$path';
     var wrong = 'package://$path';
 
     throw "URIs using the 'package:' scheme should look like "
           "'$right', not '$wrong'.";
   }
 
   var packageRoot = _packageRoot == null ?
                     _entryPointScript.resolve('packages/') :
                     _packageRoot;
-  return _filePathFromUri(packageRoot.resolve(uri.path).toString());
+  return packageRoot.resolve(uri.path);
 }
 
 
 int _numOutstandingLoadRequests = 0;
 var _httpClient;
 
 void _httpGet(Uri uri, String libraryUri, loadCallback(List<int> data)) {
   if (_httpClient == null) {
     _httpClient = new HttpClient()..maxConnectionsPerHost = 6;
   }
@@ skipping 79 matching lines @@
   if (_logBuiltin) {
     _print('# Creating uri for: $uri');
   }
 
   switch (uri.scheme) {
     case '':
     case 'file':
     case 'http':
       return uri;
     case 'package':
-      return Uri.parse(_filePathFromPackageUri(uri));
+      return _resolvePackageUri(uri);
     default:
       // Only handling file, http, and package URIs
       // in standalone binary.
       _logResolution('# Unknown scheme (${uri.scheme}) in $uri.');
       throw 'Not a known scheme: $uri';
   }
 }
 
 
 // Asynchronously loads script data through a http or file uri.
@@ skipping 61 matching lines @@
     filename = '$name.dll';
   } else {
     if (_logBuiltin) {
       _print('Native extensions not supported on ${Platform.operatingSystem}');
     }
     throw 'Native extensions not supported on ${Platform.operatingSystem}';
   }
 
   return [path, filename, name];
 }