Add a missing .url.

sdk/lib/_internal/pub/bin/async_compile.dart

 // Copyright (c) 2014, 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:io';
 
 import 'package:args/args.dart';
 import 'package:analyzer/src/services/formatter_impl.dart';
 import 'package:async_await/async_await.dart' as async_await;
 import 'package:path/path.dart' as p;
@@ skipping 158 matching lines @@
 }
 
 /// Fix relative imports to dart2js libraries.
 ///
 /// Pub imports dart2js using relative imports that reach outside of pub's
 /// source tree. Since the build directory is in a different location, we need
 /// to fix those to be valid relative imports from the build directory.
 String _fixDart2jsImports(String sourcePath, String source, String destPath) {
   var compilerDir = p.url.join(sourceUrl, "../compiler");
   var relative = p.url.relative(compilerDir,
-      from: p.dirname(p.toUri(destPath).toString()));
+      from: p.url.dirname(p.toUri(destPath).toString()));
   return source.replaceAll(_compilerPattern, "import '$relative");
 }
 
 /// Regenerate the pub snapshot from the async/await-compiled output. We do
 /// this here since the tests need it and it's faster than doing a full SDK
 /// build.
 void _generateSnapshot(String buildDir) {
   buildDir = p.normalize(buildDir);
 
   var entrypoint = p.join(generatedDir, 'bin/pub.dart');
@@ skipping 32 matching lines @@
 ///
 /// This swallows errors to accommodate multiple compilers running concurrently.
 /// Since they will produce the same output anyway, a failure of one is fine.
 void _writeFile(String path, String contents) {
   try {
     new File(path).writeAsStringSync(contents);
   } on IOException catch (ex) {
     // Do nothing.
   }
 }