[cfe] Remove best-effort testing of round-trip text serialization

Tests are now run over entire libraries instead of their sub-trees
that are deemed to be ready for the serialization by the verifier.
The update revealed two scoping bugs.  This CL fixes them as well,
namely the following:

  * Result expression of a block expression should be in the scope of
    the block.  That is, the variables declared in the block are
    visible in the result expression.

  * Local function declarations should be visible in the blocks they
    are declared in.  Accidentally, the Bind combinator was used
    instead of Rebind, resulting in local functions being visible in
    their own bodies, but not in the blocks they are declared in.

Change-Id: I856045464caa934228d5f259a4d2d582e77deb88
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155124
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>