Reland: [dart:io] Adds Socket.startConnect

This is a reland of https://dart-review.googlesource.com/c/sdk/+/62484
with the following changes:
- _NativeSocket.connect now drops references to pending sockets on
  an error or successful connection.
- eventhandlers are updated to ignore unset Dart ports on a close
  command.
- Test updated to account for new SocketException.

This is the second part of https://dart-review.googlesource.com/c/sdk/+/62484

This CL adds a startConnect method to Socket types that returns
a ConnectionTask object that can be cancelled. Cancelling
a ConnectionTask closes any sockets that were opened for the
connection attempt that are not yet connected to the host.

This allows a closing HttpClient to close sockets for pending
requests whose sockets weren't fully connected yet.

related https://github.com/flutter/flutter/issues/18617

Change-Id: I47fe3564e41197d622079aad4bb644bbdfe0bfe8
Reviewed-on: https://dart-review.googlesource.com/63040
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
17 files changed
tree: bf28406dcb06c0bfa2ddf54842b1b3bee138f93a
  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. AUTHORS
  22. BUILD.gn
  23. CHANGELOG.md
  24. codereview.settings
  25. dartdoc_options.yaml
  26. DEPS
  27. LICENSE
  28. PATENTS
  29. PRESUBMIT.py
  30. README.dart-sdk
  31. README.md
  32. WATCHLISTS
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