Inferring void as a function expression return type

[scheglov]

@eernstg there are also breakages in Flutter. Let me know if it looks like a my bug.

The spec was changed.
The analyzer change breaks following tests:

co19_2/LibTest/io/RawDatagramSocket/asBroadcastStream_A02_t02
co19_2/LibTest/io/RawDatagramSocket/asBroadcastStream_A03_t01
co19_2/LibTest/io/RawDatagramSocket/asyncExpand_A01_t01
co19_2/LibTest/io/RawDatagramSocket/asyncMap_A01_t01
co19_2/LibTest/io/RawDatagramSocket/expand_A01_t01
co19_2/LibTest/io/RawDatagramSocket/expand_A02_t01
co19_2/LibTest/io/RawDatagramSocket/expand_A02_t02

[eernstg]

There should not be any new behaviors without null-safety, cf. dart-lang/language#1092 (comment).