Flow analysis: track detailed non-promotability reasons for getters.

- When the shared logic in `field_promotability.dart` decides that a
  given field should be non-promotable based on attributes of the
  field itself (i.e. because it's public, non-final, or external), it
  returns information to the caller about the reason for
  non-promotability.

- This information is recorded by the analyzer and CFE, and delivered
  back to flow analysis in response to a new callback method,
  `whyIsPropertyNotPromotable`. Flow analysis records this information
  in the `PropertyNotPromoted`, which is delivered to clients when a
  compile-time error occurs due to a property access not being
  promotable. This ensures that the appropriate
  `http://dart.dev/go/non-promo-...` link will be associated with the
  context message. In a future CL, the context messages themselves
  will be updated to match the link.

- The shared logic in `field_promotability.dart` also collects, for
  each field name that isn't promotable, all the reasons why that
  particular field name is non-promotable, in a new data structure,
  `FieldNameNonPromotabilityInfo`. In a future CL, this data structure
  will be used to explain to the caller situations in which a field is
  non-promotable due to interference from other fields or getters with
  the same name.

Change-Id: I89ad102a4bec071bf59374971a8d83b061d4ec1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/327901
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>