Steps to Reproduce

1. Make a custom ColorScheme named customColorScheme using ColorScheme.fromSeed()
2. The seedColoris Colors.orange
3. Set surfaceTint to Colors.green using copyWith()

ColorScheme customColorScheme = ColorScheme.fromSeed(seedColor: Colors.orange)
.copyWith(
  surfaceTint: Colors.green,
);

4. Make a Container with color: customColorScheme.surfaceTint

Container(
  width: 100,
  height: 100,
  color: customColorScheme.surfaceTint,
)

Expected results:
A Container filled with Colors.green color

Actual results:
A Container filled with customColorScheme.primary color (tone 40 of Colors.orange used as seed)

Steps to solve the bug:

1. Open color_scheme.dart from flutter/packages/flutter/lib/src/material/
2. Change line 840 from
   surfaceTint: _surfaceTint ?? this.surfaceTint,
   to
   surfaceTint: surfaceTint ?? this.surfaceTint,