Dart: Better handle leak checks. close() is async.

mojo/public/dart/src/data_pipe.dart

Index: mojo/public/dart/src/data_pipe.dart
diff --git a/mojo/public/dart/src/data_pipe.dart b/mojo/public/dart/src/data_pipe.dart
index 0335cceb91556c3fe002b3e72cd0d438a01dd5dc..7f1666d319001456bf48aacfef9535dc192bc5d8 100644
--- a/mojo/public/dart/src/data_pipe.dart
+++ b/mojo/public/dart/src/data_pipe.dart
@@ -41,7 +41,7 @@ class MojoDataPipeProducer {
   int write(ByteData data, [int numBytes = -1, int flags = 0]) {
     if (handle == null) {
       status = MojoResult.INVALID_ARGUMENT;
-      return status;
+      return 0;
     }
 
     int data_numBytes = (numBytes == -1) ? data.lengthInBytes : numBytes;
@@ -49,7 +49,7 @@ class MojoDataPipeProducer {
         handle.h, data, data_numBytes, flags);
     if (result == null) {
       status = MojoResult.INVALID_ARGUMENT;
-      return status;
+      return 0;
     }
 
     assert((result is List) && (result.length == 2));