* 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.
* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.
