Previously, a ShadowSyntheticExpression was its own kind of Expression
unrelated to any of the Expression types defined in the kernel. It
responded to visit requests by forwarding the visit request to the
desugared expression. This had the advantage that for serialization
purposes, ShadowSyntheticExpressions would automatically disappear.
But it had the disadvantage that if a ShadowSyntheticExpression ever
needed to be replaced with another node (e.g. because an implicit
downcast was needed), the _ChildReplacer would get confused by the
visitor forwarding and fail to replace the node properly.
This CL fixes the problem by changing ShadowSyntheticExpression to a
Let node of the form `let _ = null in desugaredExpression`. Since
this is a genuine kernel expression, it no longer needs special
visiting semantics, and node replacement works properly.
In most circumstances, we replace the ShadowSyntheticExpression with
its desugared equivalent during type inference, so the final
serialized kernel representation is unchanged. However, in a few rare
circumstances involving error recovery, a ShadowSyntheticExpression
remains in the tree. Semantically this shouldn't be a problem, since
the behavior of `let _ = null in x` is the same as the behavior of
`x`. Nonetheless, I hope to get rid of these rare circumstances in
follow up CLs.
Change-Id: Iacf8c0028c424ac9ef98ef8302680e289bf7a017
Reviewed-on: https://dart-review.googlesource.com/21571
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Paul Berry
18fd8b75bd