PlatformIsolateManager methods should assert they're called on the platform thread #150586
Description
In PlatformIsolateManager::RemovePlatformIsolate and PlatformIsolateManager::ShutdownPlatformIsolates, find a way of asserting that we're on the platform thread. This is tricky because there may not be a current UIDartState.
This issue is a spin off from #136314