Not every month begins on a Monday (#4004)

Also, clean up the DatePicker to use more modern technology, such as a
grid for displaying the days of the month.

Fixes #3976

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

@@ -1582,7 +1582,7 @@ class CustomGrid extends GridRenderObjectWidgetBase {
 
 /// Uses a grid layout with a fixed column count.
 ///
-/// For details about the grid layout algorithm, see [MaxTileWidthGridDelegate].
+/// For details about the grid layout algorithm, see [FixedColumnCountGridDelegate].
 class FixedColumnCountGrid extends GridRenderObjectWidgetBase {
   FixedColumnCountGrid({
     Key key,