add no_top_public_members_in_executable_libraries

[a14n]

Description

Top-level members in an executable library should be private (or relocated to within main), to prevent unused members.

BAD:

main() {}
void f() {}

GOOD:

main() {}
void _f() {}

Fixes #1258

[coveralls]

Coverage increased (+0.6%) to 96.331% when pulling a99826d on a14n:no_top_public_members_in_executable_libraries into a91b233 on dart-lang:master.

[a14n]

PTAL

[bwilkerson]

The implementation looks fine; I can't speak to the question of whether this is a lint that we want to add (and that question should be discussed in the issue, not here).