Add a missing .url.

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

 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;
 final sourceDir = p.dirname(p.dirname(p.fromUri(Platform.script)));
 final sourceUrl = p.toUri(sourceDir).toString();
 final generatedDir = p.join(p.dirname(sourceDir), 'pub_generated');
 bool hadFailure = false;
 bool verbose = false;
@@ skipping 86 matching lines @@
     return result.source;
   } catch (ex) {
     stderr.writeln("Async compile failed on $sourcePath:\n$ex");
     hadFailure = true;
     return null;
   }
 }
 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()));
+      p.url.relative(compilerDir, from: p.url.dirname(p.toUri(destPath).toString()));
   return source.replaceAll(_compilerPattern, "import '$relative");
 }
 void _generateSnapshot(String buildDir) {
   buildDir = p.normalize(buildDir);
   var entrypoint = p.join(generatedDir, 'bin/pub.dart');
   var packageRoot = p.join(buildDir, 'packages');
   var snapshot = p.join(buildDir, 'dart-sdk/bin/snapshots/pub.dart.snapshot');
   var result = Process.runSync(
       Platform.executable,
       ["--package-root=$packageRoot", "--snapshot=$snapshot", entrypoint]);
   if (result.exitCode != 0) {
     stderr.writeln("Failed to generate snapshot:");
     if (result.stderr.trim().isNotEmpty) stderr.writeln(result.stderr);
     if (result.stdout.trim().isNotEmpty) stderr.writeln(result.stdout);
     exit(result.exitCode);
   }
   if (verbose) print("Created pub snapshot");
 }
 void _deleteFile(String path) {
   try {
     new File(path).deleteSync();
   } on IOException catch (ex) {}
 }
 void _writeFile(String path, String contents) {
   try {
     new File(path).writeAsStringSync(contents);
   } on IOException catch (ex) {}
 }