This is a tracking bug for the Platform Isolate feature. The design details are still being worked out, but the general idea is to provide a way of using FFI to invoke native APIs that can only be be called from the platform thread.

Some related bugs: #113099, #28310, dart-lang/sdk#52106, dart-lang/sdk#38315, dart-lang/sdk#42570, dart-lang/sdk#46943, dart-lang/native#463

Remaining work tasks after initial PR landed

• Allow calling runInPlatformThread() from helper isolates: Allow calling runOnPlatformThread() from helper isolates #150584
• In PlatformIsolateManager::RemovePlatformIsolate and PlatformIsolateManager::ShutdownPlatformIsolates, find a way of asserting that we're on the platform thread: PlatformIsolateManager methods should assert they're called on the platform thread #150586