When iterating on core library changes or changes in the AOT compiler
many seconds are wasted waiting on gen_kernel/compile_platform to
parse Dart code. This happens because we are running these tools
from sources on prebuilt Dart SDK.

This CL allows SDK developer to opt-in into AOT compiling these
tools by adding `precompile_tools=true` to their DART_GN_ARGS.

AOT compilation is performed using prebuilt SDK - so these
executables do not need to be recompiled if core libraries or
VM changes reducing iteration cycles.

pkg/vm/tool/precompiler2 is tweaked to detect when DART_GN_ARGS
contains `precompile_tools=true` and use precompiled
gen_kernel.exe instead of running it from source.

Using precompiled compile_platform takes vm_platform_strong.dill
build from 20 seconds to 3 seconds.

Using precompiled gen_kernel takes small benchmark build from
~10 seconds to 2 seconds.

This relands 5cda2a87 with fixes
for Flutter build.

TEST=manually tested

Change-Id: I552861c80c152890655e41baaf6ea3fb3b03a57e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367961


Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>