Instead of asking developer to manually press "M" during a flutter run, sometimes it's more ergonomic to automatically write SkSLs by the end of flutter run or flutter drive when --cache-sksl is provided.

The new workflow also makes it less likely to receive an empty set of SkSLs as --cache-sksl is required.

The old workflow would still be useful if developers aren't sure how to trigger a shader compilation so they can do something and press "M" iteratively.

Finally, the new workflow makes it easier for CI tests since they don't have to simulate a "M" key press.