Make Isolate.spawn work without loading from source in JIT mode

[mraleph]

Currently Isolate.spawn(func, ...) simply loads newly spawned isolate from source in JIT mode.

In the Dart 2 / Kernel world this seems to be an incorrect / partially working approach, because use cases like running from a dill file are broken as VM attempts to parse code for a newly spawned isolate using a legacy parser.

It seems strange attempt to parse the code again instead of loading it from Kernel binary.

I think we should try to solve this in two steps:

• When spawning isolate through Isolate.spawn(func, ...), spawn it from the Kernel binary (if available) that spawned original isolate;
• Figure out a way to spawn an isolate from decomposed Kernel binary available in the heap of the original isolate

/cc @kmillikin @jensjoha @kmillikin

[mkustermann]

I will try to see how hard this is.

[mkustermann]

The VM can now be launched with a dill file and use Isolate.spawn. This was the main thing I wanted to get working, so the new sim* builders we've added run isolate tests which increases our coverage there.

The remaining things, regarding not loading it from disc again but rather use the kernel blobs from the spawning isolate as well as supporting spawning from URIs are two other things I don't plan on looking into.

[mraleph]

The rest of work is going to happen under #6610 umbrella, closing this one.