Changing ButtonScheme Header to accept protocols instead of actual class.
MODIFY
components/Buttons/src/ButtonThemer/MDCButtonScheme.h
components/Buttons/src/ButtonThemer/MDCButtonScheme.h
@interface MDCButtonScheme : NSObject <MDCButtonScheming>
// Redeclare protocol properties as readwrite
-@property(nonnull, readwrite, nonatomic) MDCSemanticColorScheme *colorScheme;
-@property(nonnull, readwrite, nonatomic) MDCTypographyScheme *typographyScheme;
+@property(nonnull, readwrite, nonatomic) id<MDCColorScheming> colorScheme;
+@property(nonnull, readwrite, nonatomic) id<MDCTypographyScheming> typographyScheme;
@property(readwrite, nonatomic) CGFloat cornerRadius;
@property(readwrite, nonatomic) CGFloat minimumHeight;

Changing ButtonScheme Header to accept protocols instead of actual class. (#3488)

https://github.com/material-components/material-components-ios/pull/3488