`AsyncImportCache` runtime error when using cached `null`

[Rafer45]

AsyncImportCache uses the function putIfAbsentAsync, which uses the postfix ! on map[key] if the map has the key. However, this gives a runtime error when null values can legitimately be stored in the map. This is the case with the _canonicalizeCache, which is of type Map<Tuple2<Uri, bool>, Tuple3<AsyncImporter, Uri, Uri>?>

Instead of using the posfix !, putIfAbsentAsync should use as V, like the implementation of putIfAbsent in https://github.com/dart-lang/sdk/blob/master/sdk/lib/collection/maps.dart#L140