Signal errors and warnings for invalid returns

Implement the feature spec for checking returns against inferred
function return types:

https://github.com/dart-lang/sdk/blob/master/docs/language/informal/invalid_returns.md

with the change that an invalid return without a subexpression is
a warning, not an error.  This is because it is a warning in the
analyzer and it would be a breaking change to turn it into an
error.

The check for valid returns is moved to exactly where we handle
returns, rather than in ensureAssignable which is used as a
helper in a lot of places. A bug in type inference was fixed: we
would use `void` for the type of return without a subexpression
instead of `null`.  To accommodate that we would use a
non-standard subtyping relation for returns.  This could lead to
us inferring a return type of `void` in cases where we should
not.

Change-Id: Iee9ece9c722f47efa305f49490d3022d0bbb9f44
Reviewed-on: https://dart-review.googlesource.com/72403
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
18 files changed
tree: afb4fb216e5a0a2beeb0fa8b153a1366a232c6e1
  1. .clang-format
  2. .gitattributes
  3. .gitconfig
  4. .github/
  5. .gitignore
  6. .gn
  7. .mailmap
  8. .packages
  9. AUTHORS
  10. BUILD.gn
  11. CHANGELOG.md
  12. CONTRIBUTING.md
  13. DEPS
  14. LICENSE
  15. PATENTS
  16. PRESUBMIT.py
  17. README.dart-sdk
  18. README.md
  19. WATCHLISTS
  20. build/
  21. client/
  22. codereview.settings
  23. dartdoc_options.yaml
  24. docs/
  25. pkg/
  26. runtime/
  27. samples-dev/
  28. samples/
  29. sdk/
  30. tests/
  31. third_party/
  32. tools/
  33. utils/
README.md

Dart

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