`invoke` function in dart2wasm's JS runtime should return a promise

[eyebrowsoffire]

dart2wasm's generated JS runtime has an invoke function for the main entry point of the program. However, this basically returns immediately, even if there is more asynchronous work to be done. We should have this function return a promise, which should attempt to match the behavior of the VM command line. That is to say, the promise should be resolved when there are no more asynchronous or pending tasks to run, and it should reject if there is an uncaught exception during execution (synchronous or asynchronous).

[osa1]

This is causing test failures to be not detected when running in Chrome, because the test controller declares success if main doesn't throw:

sdk/pkg/test_runner/lib/src/test_controller.js

Lines 316 to 339 in 2534859

	function dartMainRunner(main) {
	dartPrint('dart-calling-main');
	try {
	main();
	} catch (error) {
	var stack;
	// TODO(rnystrom): Use this for script tag and window onerror calls.
	if (testErrorToStackTrace) {
	stack = testErrorToStackTrace(error);
	} else {
	// TODO(omersa): This `error.stack` is undefined when thrown from
	// dart2wasm module runner script in `dart2wasmHtml`.
	if (error.stack === undefined) {
	stack = "<undefined stack>";
	} else {
	stack = error.stack.toString();
	}
	}
	recordEvent('sync_exception', error.toString(), stack);
	notifyDone('FAIL');
	return;
	}
	dartPrint('dart-main-done');
	}

This is not a problem when running in d8 because we implement our own microtask scheduler in d8, and the same "run Dart main" function in d8 waits for the tasks before returning:

sdk/pkg/dart2wasm/bin/run_wasm.js

Lines 293 to 297 in 2534859

	self.dartMainRunner = function(main, ignored_args) {
	// Initialize.
	var action = async function() { await main(scriptArguments, null); }
	eventLoop(action);
	};