Flow analysis: break up libraries.

This change breaks flow_analysis.dart into the following libraries:

- assigned_variables.dart (for the AssignedVariables class and related
  code)

- promotion_key_store.dart (for the PromotionKeyStore class)

- type_operations.dart (for the TypeOperations mixin and related code)

- flow_analysis.dart (for the rest of flow analysis)

And it breaks mini_ast.dart into the following libraries:

- flow_analysis_mini_ast.dart (functionality specifically concerned
  with testing flow analysis)

- mini_ast.dart (functionality not specifically related to flow
  analysis)

This is in preparation for trying to share some more type inference
behaviors between the analyzer and CFE.

Note that although the diff is big, the only changes in this CL are
moving code from one place to another, renaming some class members
from private to public, and updating imports.

Change-Id: I71768f03b1e75ed754c7b7af39f6cf7f03c4fe44
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/254462
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>