[gen_snapshot] Do not add any dependency file in the kernel flow.
[gen_snapshot] Do not add any dependency file in the kernel flow.

Adding a dependency file triggers resolving its file path using the
service isolate. However, in the kernel flow, gen_snapshot does not
initialize the service isolate. This does not seem to cause any problems
on Mac and Linux, but flutter crashes on Windows. On Linux and Mac,
the behaviour with this change matches the current behavior.

The correct dependency file to add to the dependencies file in the kernel
flow is the app dill file passed as input to gen_snapshot. But, this would
call for specifyint the application sources as the dependency for the app
dill file itself somewhere. We can revisit this and fix it up correctly if
needed. For now, this change unblocks the Windows builds.

Change-Id: Ic22a7ccc96548e005b85308c3baaeedb860f4dae
Reviewed-on: https://dart-review.googlesource.com/31280
Commit-Queue: Siva Chandra <85550dee2503cc6ca35639514fa03c6933752df4@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

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