This CL makes the following improvements:
- Makes ParameterElementImpl._unlinkedParam public so that it can be
used by the ExprBuilder.
- Adds logic to ExprBuilder to create elements for closure parameters
and to resolve references to those elements.
- Adds support for async closures.
- Adds support for nested closures.
Change-Id: I4b2c2271dac44bb54eb12049de7143b14adef84a
Reviewed-on: https://dart-review.googlesource.com/48680
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
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