[Patterns] Flow analysis

[stereotype441]

Tracking issue for the implementation of flow analysis for patterns.

This includes implementing the flow analysis engine for patterns and adding calls to the engine from the shared logic in pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer.dart. I'm assuming that the analyzer and CFE will use that shared logic for all their patterns analysis, so there shouldn't be any need to add calls from the analyzer and CFE directly to flow analysis for pattern support.

[stereotype441]

I'm assuming I'll be doing a lot of this work, so I'm assigning to myself. So far I've written up an implementation plan but no actual code.

If anyone would like to collaborate on this, please let me know.

[bwilkerson]

I marked this P1 because some priority is part of the analyzer issue triage process and it's related to a language feature.