[vm] Remove BaseIsolate as it serves no purposes anymore.

It was created to improve StackResource construction/destruction https://chromiumcodereview.appspot.com//10008030, but then StackResource constructor was refactored away from using BaseIsolate.

TEST=ci

Change-Id: Ie2e1ac1d04210032f33cbfd6c8e0827ac745e0b3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401540
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam