[dart:js_interop] Add isA helper

Closes https://github.com/dart-lang/sdk/issues/54138

Adds a helper to do better type-checks so that users don't
accidentally using is checks or have to manually do the right
typeof or instanceof checks. In order to do this, there is
some refactoring to make ExportCreator a SharedInteropTransformer
(as it's shared across all backends) so that we can reuse an
existing visitor. In the same class, we remove unnecessary setting
of parent pointers. We should clean up the fileOffsets as well,
but dart2js verifies that those are set, so we keep them as is
for now. Also adds some static errors for edge cases.

CoreLibraryReviewExempt: Helper for web-specific library.
Change-Id: I34d818ada1349b69afd15d170d3fafa0460f65fa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/347225
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>