Flow analysis: add field promotion support for cascades.

This change updates the flow analysis support for field promotion
(which is not yet switched on by default) so that it supports field
accesses inside cascade expressions. The key moving parts are:

- The type hierarchy `PropertyTarget` (which is used by the client to
  tell flow analysis whether the target of a property access is
  `this`, `super`, or an ordinary expression) now has a new class,
  `CascadePropertyTarget`, to represent the situation where the target
  of the property access is an implicit reference to the target of the
  innermost enclosing cascade expression.

- Flow analysis has two new methods on its API:
  `cascadeExpression_afterTarget` and `cascadeExpression_end`, so that
  the client can inform flow analysis when a cascade expression is
  being analyzed.

- Flow analysis uses its `_makeTemporaryReference` method to track the
  implicit temporary variable that stores the target of cascade
  expressions. (This method was developed as part of flow analysis
  support for patterns, where it creates the data structures necessary
  to track the implicit variables that are created as part of pattern
  desugaring).

- The "mini-AST" pseudo-language used by flow analysis unit tests now
  has a way to represent cascade expressions and method invocations.

- In addition to unit tests for `_fe_analyzer_shared`, `analyzer`, and
  `front_end`, there are new language tests in
  `tests/language/inference_update_2` to test cascaded field
  promotions in end-to-end fashion.

Bug: https://github.com/dart-lang/language/issues/2020
Change-Id: I21353bbc884ed599cb1739cecfb68ad1d975d18b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309220
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
25 files changed
tree: 91838ca0e95584102a5ac634231e5b125b540200
  1. .dart_tool/
  2. .github/
  3. benchmarks/
  4. build/
  5. docs/
  6. pkg/
  7. runtime/
  8. samples/
  9. sdk/
  10. tests/
  11. third_party/
  12. tools/
  13. utils/
  14. .clang-format
  15. .gitattributes
  16. .gitconfig
  17. .gitignore
  18. .gn
  19. .mailmap
  20. .style.yapf
  21. .vpython
  22. AUTHORS
  23. BUILD.gn
  24. CHANGELOG.md
  25. codereview.settings
  26. CONTRIBUTING.md
  27. DEPS
  28. LICENSE
  29. OWNERS
  30. PATENT_GRANT
  31. PRESUBMIT.py
  32. README.dart-sdk
  33. README.md
  34. sdk.code-workspace
  35. sdk_args.gni
  36. SECURITY.md
  37. WATCHLISTS
README.md

Dart

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