Set modal bottom sheet barrier color

[katallaxie]

Description

When presenting a Modal Bottom Sheet this PR allows to pass down a value for the color of the modal barrier. This is a non-breaking change, because the default behavior is being kept.

Tests

I added the following tests:

• A test which confirms that the presented modal barrier color reflects the passed in value for the color.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[katallaxie]

@darrenaustin any thoughts here. Has been laying around for a bit.

[darrenaustin]

Thanks for your contribution. This is a reasonable feature with a good implementation and test. LGTM.

[darrenaustin]

@darrenaustin any thoughts here. Has been laying around for a bit.

Sorry about that. Looks good. I will push this when the build goes green. Thanks for submitting this.

[droidery]

@katallaxie Shouldn't this also be exposed in the BottomSheetThemeData. This would allow the same modal sheet barrier color to be used across the entire app.

It would also be more consistent with the other properties available in the BottomSheetThemeData like backgroundColor and shape.

[katallaxie]

@droidery I thought of this. But because there is default behavior that actually reflects the standard design guidelines I thought it would be better to have this more as a special customization.