MinimumTextContrastGuideline fails when button/label contrast appears to be OK #22044
Description
I've introduced some significant changes to ButtonTheme et al. in #22013.
With these changes, three RaisedButton contrast tests from examples/flutter_gallery/test/accessibility_test.dart fail.
The buttons appear to be rendered correctly and with sufficient contrast. The tests computed foreground and background colors aren't what I'd expect, they should be background: 0xff0175c2 (see examples/flutter_gallery/lib/gallery/themes.dart primaryColor in buildLightTheme), foreground: white.
// 'modal_bottom_sheet_demo'
The following TestFailure object was thrown running a test:
Expected: Text contrast should follow WCAG guidelines
Actual: <Instance of 'WidgetTester'>
Which: SemanticsNode#786(Rect.fromLTRB(265.0, 304.0, 535.0, 352.0), actions: [tap], flags:
[isButton, hasEnabledState, isEnabled], label: "SHOW BOTTOM SHEET", textDirection: ltr):
Expected contrast ratio of at least 4.5 but found 2.65 for a font size of 14.0. The
computed foreground color was: Color(0xff6200ee), The computed background color was:
Color(0xff0d0020)
See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
// 'persistent_bottom_sheet_demo'
The following TestFailure object was thrown running a test:
Expected: Text contrast should follow WCAG guidelines
Actual: <Instance of 'WidgetTester'>
Which: SemanticsNode#893(Rect.fromLTRB(279.0, 168.0, 521.0, 216.0), actions: [tap], flags:
[isButton, hasEnabledState, isEnabled], label: "SHOW A SNACKBAR", textDirection: ltr):
Expected contrast ratio of at least 4.5 but found 2.65 for a font size of 14.0. The
computed foreground color was: Color(0xff6200ee), The computed background color was:
Color(0xff0d0020)
See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
// 'snack_bar_demo'
The following TestFailure object was thrown running a test:
Expected: Text contrast should follow WCAG guidelines
Actual: <Instance of 'WidgetTester'>
Which: SemanticsNode#786(Rect.fromLTRB(265.0, 304.0, 535.0, 352.0), actions: [tap], flags:
[isButton, hasEnabledState, isEnabled], label: "SHOW BOTTOM SHEET", textDirection: ltr):
Expected contrast ratio of at least 4.5 but found 2.65 for a font size of 14.0. The
computed foreground color was: Color(0xff6200ee), The computed background color was:
Color(0xff0d0020)
See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html