Make LUB/GLB more useful for generic types

[jmesserly]

consider:

void main() {
  var l = true ? [] : <int>[];
  l.add(42);
  print(l);
}

This l is inferred as EfficientLength

(see dart-archive/dev_compiler#288)

Edit: and also fix GUB, as @leafpetersen notes in this comment.

[jmesserly]

CC @natebosch, who just hit this again (in #26079). @munificent @leafpetersen were we thinking of looking at this in the short term?

[leafpetersen]

It's on the list of things that I really want to fix. A full fix probably takes more time than it's worth in the immediate short term though. If I can find the time in the next few weeks I'll at least try to handle the obvious cases.