[kernel] Implement visitExtensionReference in VisitorThrowingMixin
[kernel] Implement visitExtensionReference in VisitorThrowingMixin

Change-Id: I472aaae83b360c31edfc29e8c47e9885a72779f4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191405
Auto-Submit: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>
Commit-Queue: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>
Commit-Queue: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk