Clean up the file names in the painting/ library (#12217)

After all my recent refactorings, some of the names were not really reasonable any more.

packages/flutter/lib/painting.dart

@@ -18,20 +18,20 @@
 library painting;
 
 export 'src/painting/basic_types.dart';
-export 'src/painting/border.dart';
 export 'src/painting/border_radius.dart';
+export 'src/painting/borders.dart';
+export 'src/painting/box_decoration.dart';
 export 'src/painting/box_fit.dart';
-export 'src/painting/box_painter.dart';
 export 'src/painting/box_shadow.dart';
 export 'src/painting/colors.dart';
 export 'src/painting/decoration.dart';
 export 'src/painting/edge_insets.dart';
 export 'src/painting/flutter_logo.dart';
 export 'src/painting/fractional_offset.dart';
+export 'src/painting/geometry.dart';
 export 'src/painting/gradient.dart';
-export 'src/painting/image.dart';
+export 'src/painting/images.dart';
+export 'src/painting/matrix_utils.dart';
 export 'src/painting/text_painter.dart';
 export 'src/painting/text_span.dart';
 export 'src/painting/text_style.dart';
-export 'src/painting/transforms.dart';
-export 'src/painting/utils.dart';

packages/flutter/lib/src/painting/box_decoration.dart

@@ -9,13 +9,13 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 import 'basic_types.dart';
-import 'border.dart';
 import 'border_radius.dart';
+import 'borders.dart';
 import 'box_shadow.dart';
 import 'decoration.dart';
 import 'edge_insets.dart';
 import 'gradient.dart';
-import 'image.dart';
+import 'images.dart';
 
 /// An immutable description of how to paint a box.
 ///

packages/flutter/lib/src/painting/geometry.dart

@@ -10,11 +10,11 @@ import 'basic_types.dart';
 
 const double _kScreenEdgeMargin = 10.0;
 
-/// Position a box either above or bellow a target box specified in the global
+/// Position a box either above or below a target box specified in the global
 /// coordinate system.
 ///
-/// The target box is specified by [size] and [target] and the box being
-/// positioned is specified by [childSize]. [verticalOffset] is the amount of
+/// The target box is specified by `size` and `target` and the box being
+/// positioned is specified by `childSize`. `verticalOffset` is the amount of
 /// vertical distance between the boxes.
 ///
 /// Used by [Tooltip] to position a tooltip relative to its parent.

packages/flutter/lib/src/painting/images.dart

@@ -8,7 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 import 'basic_types.dart';
-import 'border.dart';
+import 'borders.dart';
 import 'box_fit.dart';
 import 'fractional_offset.dart';