no events from MimeMultipart.listen() when called asynchronously

[DartBot]

Originally opened as dart-lang/sdk#20140

This issue was originally filed by [email protected]

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What steps will reproduce the problem?

1. Create an HTTP server
2. Transform requests with MimeMultipartTransformer
3. listen() on the resulting MimeMultipart object after a short Timer callback

What is the expected output? What do you see instead?

I expect to get data events the way I should from listen(). Instead, I usually get zero callbacks. If I was uploading a large file, I sometimes get one empty array. It is not very impressive.

What version of the product are you using?

The mime library version 0.9.0+3, Dart version 1.5.3.

On what operating system?

Mac OS X

What browser (if applicable)?

N/A

Please provide any additional information below.

I have recreated this bug in an isolated project here: https://github.com/unixpickle/dart-multipart-bug. Additionally, that repository includes a patch which fixes the issue. Somebody else should look over my patch before verifying it, but it's pretty small.