Better context message on SyntheticForwarder in MissingImplementation

Previously we could get something like this:
```
regress_32660_test_06.dart:9:7: Context: 'foo' is defined here.
class I extends G implements H {
      ^^^
```

because a SyntheticForwarder was added with a file location of the
class.

It now instead goes to the forwarding stub target and says
```
regress_32660_test_06.dart:6:3: Context: 'foo' is defined here.
  foo(int x, {int y}) => y;
  ^^^
```

Change-Id: I6fb1b958f58635c9afffa8e58cd49eb69df28afd
Reviewed-on: https://dart-review.googlesource.com/72680
Reviewed-by: Peter von der Ahé <ahe@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
7 files changed
tree: 78542da4bd400a51445935a14e45a5933dac2548
  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/
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