The new async iterable type broke LL(1) rule

[saschanaz]

What is the issue with the Web IDL Standard?

After #1397 the grammar is not LL(1) because it requires more lookahead:

async iterable<unsigned long long> foo();
//                                 ^ Heh, dear parser, you expected AsyncIterable? Too bad!

See also the build error in #1489 (comment).

 The WebIDL grammar is NOT LL(1) due to the following conflicts:
    1. PartialInterfaceMember:
       * "async" (54/58)

cc @lucacasonato