support hot reload from a partial kernel file

[sigmundch]

(I couldn't find whether we have filed an issue for this already)

Hot reload from kernel works when the incremental kernel generator sends down the entire program after a reload. However, if we send a partial program, the VM crashes.

To reproduce:

• remove the skip: true in hot_reload_e2e_test
• then run

./out/ReleaseX64/dart pkg/front_end/test/src/incremental/hot_reload_e2e_test.dart

you should see the failure on the unit test titled "reload after non-leaf library modification".

You can also run this test via test.py, but you need to remove the Skip entry in the pkg.status file at this time. The test is being skipped because it appears to show other flaky issues as well (see #29819)

[sigmundch]

Note: because the test is being skipped, I noticed that the test broke when we removed the --platform flag. This CL https://codereview.chromium.org/2976553002/ should fix that.

[sigmundch]

Note: the CL landed, so the test is now updated and operational in master (but still marked as Skip in the status file)