[web] Support for running native tests in dartdevc.

This consists of a few combined changes:
* support `@Native` on tests: This entails calling registerExtensions
  on user code, but only when running on native tests (which are explicitly
  added to an allow list).

* support for invoking external top-level methods also on native tests.

* support for running `applyExtension` later in tests. This is because
  tests introduce the JavaScript definitions after the calls to
  registerExtensions happen. Note: this wouldn't be necessary if we one
  day change how tests are loaded (e.g. if we can load the JavaScript
  portion of the test before the module compiled by dartdevc is loaded).
  That would entail more infrastructure work and given that this is only
  used for ~200 tests, it doesn't seem worthwhile at this time.

* ensure extension symbols are also stored in `dartx.*` from a native
  test. This is to ensure dynamic calls are fully supported.

* changes to the dart2js runtime to ensure the test code can run in both
  dartdevc and dart2js with the same API.

* one updated test: all native tests need to be updated and I'll follow
  up with those, but the changes on this test are included in this CL to
  illustrate how this all fits together.

After this change (and parent changes), you can now run:

  ./tools/test.py -m release -c dartdevk -r d8 dart2js/native/native_checked_arguments1_frog_test

Change-Id: I4c2615a80b0e7611aa6f57abaa55b4e03b42bfe4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177180
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>