Don't pass MultiFrameCodec across threads

[amirh]

The MultiFrameCodec::getNextFrame which is running on the UI thread is passing "this" as part of a closure to the IO thread.
This is unsafe, as it is possible for posted task to run on the IO thread after the dart isolate quit and cleared the object.

@chinmaygarde

[Hixie]

cc @chinmaygarde did we ever fix this? :-)

[chinmaygarde]

Just took a look at it, and, no. It's pretty hairy. We should prioritize a fix for this ASAP.

[Hixie]

@chinmaygarde Who would be the best person to look at it?

[Hixie]

cc @dnfield

[chinmaygarde]

This isn’t particularly difficult. Anyone with spare cycles to cleanup the callback spaghetti and add tests now that we have harness for it could take a stab at this. Is this showing up in a customer issue?