Shared type analysis: rework docs and add tests

Update the documentation for TypeAnalyzer and related classes so that
they explain the behavior of each method in terms of its effect on a
stack.  (The test logic already contained an implementation of such a
stack; production clients may or may not need to keep a stack
depending on their requirements).  This should make it more
straightforward to write clients of TypeAnalyzer.

Also, beef up the test logic in flow_analysis_mini_ast.dart and so that:

- In the event of a test error, a source location is shown, so that
  it's easy to debug and/or update the test.  This source location is
  obtained by parsing `StackTrace.current`, an approach which I
  wouldn't recommend for production code, but which is servicable for
  these low-level tests.

- Items that are popped off the stack are checked to make sure they
  have the expected kind.

The kind-checking caught a minor flaw in the previous test logic: it
was failing to distinguish expressions from expression statements.
This as been corrected, and as a result, a few test expectations in
type_inference_test.dart needed to be updated.

Change-Id: I0a2b257f6e970478c0c8e1b663dd5a367e7f24ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/257486
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>