Background

flutter/engine#36538 attempted to update the engine to Visual Studio 2019, however this was reverted as it broke the framework tree.

Problem

The root cause is that the EventChannel's handler stores and mutates data on the EventChannel itself. This is problematic as the EventChannel's and the handler's lifetimes are decoupled (the EventChannel does not own its handler). If the EventChannel is destroyed, the handler reads/mutates deallocated memory.

See this discussion for additional background info: #113135 (comment)