[ VM / CLI ] Redirect users to use 'dart compile' for snapshot generation
[ VM / CLI ] Redirect users to use 'dart compile' for snapshot generation

Note: `--snapshot=` and related flags are still valid for non-DartDev
invocations of the VM (e.g., 'dart --snapshot=').

TEST=Local testing

Fixes https://github.com/dart-lang/sdk/issues/49276

Change-Id: I666f68bd46d5b22c691da15267f14f4ad5a53731
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249220
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk