* Read core platform kernel file during Dart initialization.

Currently service isolate is initialized from the source code parsed by VM.
This CL changes it so service isolate created during Dart initialization
is created from the kernel platform.dill file if it is present in the application
bundle. Then this platform kernel file is kept in dart_init module and reused
for application sciprt isolates.

* Reformat and merge

* Use accessor method

* Avoid passing running_from_kernel param. Add TODO for cleanup. Rename param.