Enforce laziness of deferred libraries in dev_compiler

[munificent]

As of 8daf4cf, dev_compiler now supports the loadLibrary() function on deferred libraries. However, it does not validate that you call that before the library is used. This currently runs:

import "something.dart" deferred as something;

main() {
  // No loadLibrary() call.
  something.someFunction();
}

It would be great if dev_compiler threw an error here. Otherwise, users can iterate on programs that work fine in dev_compiler but fail when a production build is run.

[Markzipan]

This should be fixed as of https://dart-review.googlesource.com/c/sdk/+/177620!