Add migration support for named parameters.

Currently assuming that the language team chooses option 2 for
https://github.com/dart-lang/language/issues/156#issuecomment-460525075.

To handle the case where the user does not specify a default value,
and all call sites supply the named parameter, there are two possible
heuristics:

- assumeNullable: mark the parameter as nullable, since the implicit
  default is `null`, even though the implicit default is never used.

- assumeRequired: consider the parameter to be `@required`, ensuring
  that the implicit default will never be used.

I've provisionally chosen `assumeRequired` as the default behavior.

Functionality not yet implemented:

- If `@required` is already present in the source code, this should
  override the heuristic; also it should not be added again.

- If the parameter is already marked as nullable in the source code,
  this should override the heuristic.

- Uses of `@required` should be translated to `required`.

- If `@required` is already present in the source code, and a default
  value is supplied, the default value should be removed.

- If the parameter is used in a way that demonstrates it was intended
  to be non-nullable (i.e. it appears in a place in unconditional
  control flow that would cause an exception to be thrown if it was
  null), this should override the heuristic and cause it to be marked
  as `@required`.

Change-Id: I9f2ec63787fdb6215e6075ecfcc830ed9942b8b5
Reviewed-on: https://dart-review.googlesource.com/c/92532
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
8 files changed
tree: b0dede16ea2f82e78be027c8268cfcfe11fda846
  1. .github/
  2. build/
  3. client/
  4. docs/
  5. pkg/
  6. runtime/
  7. samples/
  8. samples-dev/
  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. .packages
  21. .vpython
  22. AUTHORS
  23. BUILD.gn
  24. CHANGELOG.md
  25. codereview.settings
  26. CONTRIBUTING.md
  27. DEPS
  28. LICENSE
  29. PATENTS
  30. PRESUBMIT.py
  31. README.dart-sdk
  32. README.md
  33. WATCHLISTS
README.md

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