fix(material/schematics): don't drop prebuilt imports in theming API migration

[crisbeto]

Currently the theming API migration drops any imports starting with ~@angular/material/, assuming that they're Sass APIs. This can result in prebuilt style imports being removed by mistake.

These changes add a regex based on which we'll exclude some imports from the migration.

Fixes #22697.

[Splaktar]

Thank you for the quick PR on this!

[Splaktar]

It should be possible to migrate these to the following

Suggested change

	`@import '~@angular/material/prebuilt-themes/indigo-pink.css';`,
	`@import '~@angular/cdk/overlay-prebuilt.css';`,
	`@use '@angular/material/prebuilt-themes/indigo-pink.css';`,
	`@use '@angular/cdk/overlay-prebuilt.css';`,

Or is that something that you are depending upon the sass-migrator to do?

[crisbeto]

I believe that imports of .css files are still supposed to go through @import.

[Splaktar]

I thought that too at first, but found that they work fine with @use as long as the deprecated sass-loader feature of using ~ is not used.

If you've seen an official statement from the Sass team to the otherwise, please do let me know.

[intellix]

I'm not seeing any migrations/automatic updates happening when updating from 11 to 12 and I can't see any information anywhere as to when an update comes with a migration or not.

Is there a list somewhere? There's no mention of it in the release notes for example. I don't know if I should just manually go through my SCSS and update the references or if it should be automatic.

The release notes just seem to suggest re-reading the theming docs and doing it yourself:

For more information, check out the new theming guide.

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