debugDrainErrors method
Everything the browser complained about since the last drain, or null when it complained about nothing.
Asynchronous because the verdicts are: every scope _guard opened is a
promise, and this waits for the ones outstanding before answering. A test
that draws and then asks gets the truth, and takes what it read: the next
beginFrame does not throw it again. Production does not need to ask;
beginFrame hands the same complaints over as a typed refusal.
Implementation
Future<String?> debugDrainErrors({String where = ''}) =>
_ledger.debugDrain(where);