Fix API misuse bug I introduced when fixing another API misuse bug. :-)

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 0fb9b9bd2643f84a6692db18118558264b955679..ad50c653375cc2629e65b85db8c66f487cd16805 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -612,15 +612,15 @@ Future<bool> extractTarGz(InputStream stream, destination) {
       ["--extract", "--gunzip", "--directory", destination]);
   var completer = new Completer<int>();
 
+  process.onExit = completer.complete;
+  process.onError = completer.completeException;
+
   // Wait for the process to be fully started before writing to its
   // stdin stream.
   process.onStart = () {
     stream.pipe(process.stdin);
     process.stdout.pipe(stdout, close: false);
     process.stderr.pipe(stderr, close: false);
-
-    process.onExit = completer.complete;
-    process.onError = completer.completeException;
   };
 
   return completer.future.transform((exitCode) => exitCode == 0);