Finalize and specify rules for local variable inference

[munificent]

The type inference rules for local variables declared without a type are pretty well-tested at this point, but I think there are a couple of corners to hammer out and then we need to add them to the spec.

Edit by @eernstg Feb 2018: Note that this is now essentially one of the subtasks of #31228.

[floitschG]

Status update:
Ideally we would like to spec the rules for 1.50, but no other team should be affected, if this issue slips.

[leafpetersen]

started: I am working on this in the background, but as noted above, this is purely a spec issue with no implementation dependencies, so lower priority.

[eernstg]

No Dart 2 milestone: This is now considered a subtask of #31228, which has a Dart 2 milestone.

[srawlins]

I think this can be closed in favor of the local variable inference rules being written for Null Safety Dart.