Add support for platform specific communication channels

[natebosch]

Some test platforms want to expose new runtime capabilities that coordinate between the code under test and the test platform. We currently implement a similar capability baked in to the test runner instead of individual platforms - spawnHybridCode and spawnHybridUri.

test/pkgs/test_core/lib/src/runner/runner_test.dart

Lines 73 to 79 in b9c59ea

	case 'spawn-hybrid-uri':
	// When we kill the isolate that the test lives in, that will close
	// this virtual channel and cause the spawned isolate to close as
	// well.
	spawnHybridUri(msg['url'] as String, msg['message'], suite).pipe(
	testChannel.virtualChannel((msg['channel'] as num).toInt()));
	break;

We can likely use the existing multichannel mechanism to create additional communication channels.

test/pkgs/test_api/lib/src/scaffolding/spawn_hybrid.dart

Lines 155 to 170 in b9c59ea

	var channel = Zone.current[#test.runner.test_channel] as MultiChannel?;
	if (channel == null) {
	throw UnsupportedError("Can't connect to the test runner.\n"
	'spawnHybridUri() is currently only supported within "dart test".');
	}
	ensureJsonEncodable(message);
	var virtualChannel = channel.virtualChannel();
	StreamChannel isolateChannel = virtualChannel;
	channel.sink.add({
	'type': 'spawn-hybrid-uri',
	'url': uri,
	'message': message,
	'channel': virtualChannel.id
	});

We will need to solve the API on the platform definition side as well.