Two-way bind @ syntax breaks one-way bind

Perhaps two-way bind @ syntax (i.e. value="@{myprop}") is not supported yet, but it doesn't even work for one-way binding; it won't update at all when the source changes. (Broken in JS and SWF, works fine in Flex.)

See attached example.
[TestTwoWayBinding.mxml.txt](https://github.com/apache/royale-asjs/files/7448669/TestTwoWayBinding.mxml.txt)

Start by reading and running the attached TestTwoWayBinding.mxml.txt example in the JS and SWF targets, then trace how value="@{myprop}" is handled when the source changes. Done means the one-way binding updates in JS and SWF, while preserving the existing working Flex behavior.

Written by the indexing model from the issue text.