Updated ColorScheme.dark() colors to match the Material Dark theme specification

[darrenaustin]

Description

According the the Material Dark theme spec the default dark color scheme's primaryVariant, surface, background and error colors should be:

Primary Variant: #3700B3 
Surface: #121212
Background: #121212
Error: #CF6679

Which are different than the current implementation of ColorScheme.dark(). As this is supposed to be the default dark theme for applications to use, it should use the recommended colors.

This PR updates ColorScheme.dark() to the match the spec colors.

Related Issues

Fixes: #36021

Tests

Added a new color_scheme_test.dart that checks the light and dark scheme against the spec values.

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).

If you depended on ColorScheme.dark() to provide the current implementation's colors, then you can just use copyWith to set those colors back to the original non-spec colors using the following:

ColorScheme.dark().copyWith(
  primaryVariant: const Color(0xff4b01d0),
  surface: Colors.black,
  background: Colors.black,
  error: const Color(0xffb00020),
)

See the announcement email for more details.

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

Reviewers: Read the Tree Hygine page and make sure this patch meets those guidelines before LGTMing.

/cc @dnfield

[HansMuller]

Please add a pair of (trivial) tests that verify the default light and dark color scheme colors per the spec.

[rami-a]

LGTM!