[dart2js] Cleanup NativeEnqueuer.
[dart2js] Cleanup NativeEnqueuer.

This cl removes an unnecessary concrete interface, and makes the
existing base class abstract.

Change-Id: I41891a7b3c57008de05a7c4a20a70452983ab441
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232125
Reviewed-by: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk