Move Brightness back to the material library (#4635)

packages/flutter/lib/src/material/theme.dart

@@ -7,7 +7,7 @@ import 'package:meta/meta.dart';
 
 import 'theme_data.dart';
 
-export 'theme_data.dart' show ThemeData;
+export 'theme_data.dart' show Brightness, ThemeData;
 
 /// The duration over which theme changes animate.
 const Duration kThemeAnimationDuration = const Duration(milliseconds: 200);

packages/flutter/lib/src/material/theme_data.dart

@@ -4,12 +4,25 @@
 
 import 'dart:ui' show Color, hashValues;
 
-import 'package:flutter/widgets.dart';
-
 import 'colors.dart';
 import 'icon_theme_data.dart';
 import 'typography.dart';
 
+/// Describes the contrast needs of a color.
+enum Brightness {
+  /// The color is dark and will require a light text color to achieve readable
+  /// contrast.
+  ///
+  /// For example, the color might be dark grey, requiring white text.
+  dark,
+
+  /// The color is light and will require a dark text color to achieve readable
+  /// contrast.
+  ///
+  /// For example, the color might be bright white, requiring black text.
+  light,
+}
+
 // Deriving these values is black magic. The spec claims that pressed buttons
 // have a highlight of 0x66999999, but that's clearly wrong. The videos in the
 // spec show that buttons have a composited highlight of #E1E1E1 on a background

packages/flutter/lib/src/widgets/basic.dart

@@ -382,21 +382,6 @@ class ClipPath extends SingleChildRenderObjectWidget {
   }
 }
 
-/// Describes the contrast needs of a color.
-enum Brightness {
-  /// The color is dark and will require a light text color to achieve readable
-  /// contrast.
-  ///
-  /// For example, the color might be dark grey, requiring white text.
-  dark,
-
-  /// The color is light and will require a dark text color to achieve readable
-  /// contrast.
-  ///
-  /// For example, the color might be bright white, requiring black text.
-  light,
-}
-
 
 // POSITIONING AND SIZING NODES