Always use persistent handles when passing dart objects as userdata to callbacks

[nielsenko]

Fixes: #1492

[coveralls-official]

Pull Request Test Coverage Report for Build 7660137927

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on kn/rid-us-of-weak-handles at 87.374%

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Totals
Change from base Build 7654644143:	87.4%
Covered Lines:	3723
Relevant Lines:	4261

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💛 - Coveralls

[nielsenko]

I have not added new unit tests for this, as it needs to run fairly long in order to provoke it, and in most cases it will hang the notifications instead of crashing the process. But the re-production code is in the issue #1492.

[nirinchev]

This looks good - is there a way to run a test to see if we leak any resources with this change? (I don't think we should but it's the kind of change that carries this risk)

[nielsenko]

This looks good - is there a way to run a test to see if we leak any resources with this change? (I don't think we should but it's the kind of change that carries this risk)

We could look into using the vmService like I do here: https://gist.github.com/nielsenko/3f3cda354931d9179d1028e3df03340d.

Or perhaps better to use the fairly new leak_tracker package

I have created a separate issue for that #1494