Flow analysis: don't necessarily clear promotions on assignment.

If the RHS of an assignment is a subtype of the current promoted type
(or one of the previously promoted types), any relevant promotions are
kept.

So for example:

  if (x is int) {
    x = x + 1; // Still an int!
    print(x.isEVen); // ok
  }

This required two significant changes:

- Clients must now tell the flow analysis engine the type of the RHS
  of the assignment.  This required some refactoring in the analyzer,
  because this information wasn't always easily available at the point
  where we were calling flow analysis.

- The flow analysis engine now has to keep track of a "chain" of the
  currently active promotions, so that it can potentially un-do some
  promotions but not others.  This meant that the algorithms for
  "join" and "restrict" had to be reworked.

In a future CL I'll add the ability for assignments to promote to
types that have been previously checked against ("types of interest"),
e.g.:

  if (x is! int) {
    x = 0; // x is now an int!
  }
  // x is now known to be an int because it's promoted in both branches

Change-Id: I63d6c9a2021b045d391b2d9c674e9a2e5f770e62
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123003
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>