Since we removed our operator== overload, we can now allow people to put
these in collections again.

Also, turn on two more analyzer lints: avoid_empty_else and hash_and_equals.

Added missing @override annotations (as per the `annotate_overrides` lint rule).

This patch makes it easier to use the auto layout API:

* We no longer use operator== because that requires an ugly cast by the
  API user.
* Also, "leftEdge" is now just "left" for less verbosity.
* AutoLayoutChild not implies its key from the AutoLayoutParam object.
* We now correctly layout every child of a RenderAutoLayout object even
  if the solver doesn't flush any updates to that child.

Allow constraint creation from multiplication and division when at least one argument is a constant expression