Create an explicit lambda for onExit.

utils/pub/io.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.
 
 /// Helper functionality to make working with IO easier.
 library io;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
@@ skipping 761 matching lines @@
   log.fine("Extracting .tar.gz stream to $destination.");
 
   if (Platform.operatingSystem == "windows") {
     return _extractTarGzWindows(stream, destination);
   }
 
   var completer = new Completer<int>();
   var processFuture = startProcess("tar",
       ["--extract", "--gunzip", "--directory", destination]);
   processFuture.then((process) {
-    process.onExit = completer.complete;
+    process.onExit = (exitCode) => completer.complete(exitCode);
     stream.pipe(process.stdin);
     process.stdout.pipe(stdout, close: false);
     process.stderr.pipe(stderr, close: false);
   }).catchError((e) {
     completer.completeError(e.error, e.stackTrace);
   });
 
   return completer.future.then((exitCode) {
     log.fine("Extracted .tar.gz stream to $destination. Exit code $exitCode.");
     // TODO(rnystrom): Does anything check this result value? If not, it should
@@ skipping 171 matching lines @@
 Directory _getDirectory(entry) {
   if (entry is Directory) return entry;
   return new Directory(entry);
 }
 
 /// Gets a [Uri] for [uri], which can either already be one, or be a [String].
 Uri _getUri(uri) {
   if (uri is Uri) return uri;
   return new Uri.fromString(uri);
 }