Updating Cupertino Button MinSize to comply with iOS guidelines (#8881)

The recommended minimum tappable area is actually 44.0pt according to the [iOS Human Interface Guidline](https://developer.apple.com/ios/human-interface-guidelines/visual-design/layout/).

> Provide ample spacing for interactive elements. Try to maintain a minimum tappable area of 44pt x 44pt for all controls.

I've also added an override to allow for flexibility in situations where designers get naughty and have a desire to make everything super small.

packages/flutter/lib/src/cupertino/button.dart

@@ -46,6 +46,7 @@ class CupertinoButton extends StatefulWidget {
     @required this.child,
     this.padding,
     this.color,
+    this.minSize: 44.0,
     @required this.onPressed,
   });
 
@@ -69,6 +70,16 @@ class CupertinoButton extends StatefulWidget {
   /// If this is set to null, the button will be disabled.
   final VoidCallback onPressed;
 
+  /// Minimum size of the button.
+  ///
+  /// Defaults to 44.0 which the iOS Human Interface Guideline recommends as the
+  /// minimum tappable area
+  ///
+  /// See also:
+  ///
+  /// * <https://developer.apple.com/ios/human-interface-guidelines/visual-design/layout/>
+  final double minSize;
+
   /// Whether the button is enabled or disabled. Buttons are disabled by default. To
   /// enable a button, set its [onPressed] property to a non-null value.
   bool get enabled => onPressed != null;
@@ -129,11 +140,13 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
       onPointerDown: enabled ? _handleTapDown : null,
       onPointerUp: enabled ? _handleTapUp : null,
       onPointerCancel: enabled ? _handleTapCancel : null,
-
       child: new GestureDetector(
         onTap: config.onPressed,
         child: new ConstrainedBox(
-          constraints: const BoxConstraints(minWidth: 48.0, minHeight: 48.0),
+          constraints: new BoxConstraints(
+            minWidth: config.minSize,
+            minHeight: config.minSize,
+          ),
           child: new FadeTransition(
             opacity: new CurvedAnimation(
               parent: _animationController,

packages/flutter/test/cupertino/button_test.dart

@@ -13,7 +13,7 @@ const TextStyle testStyle = const TextStyle(
 );
 
 void main() {
-  testWidgets('Layout minimum size', (WidgetTester tester) async {
+  testWidgets('Default layout minimum size', (WidgetTester tester) async {
     await tester.pumpWidget(
       new Center(child: new CupertinoButton(
         child: new Text('X', style: testStyle),
@@ -23,8 +23,25 @@ void main() {
     final RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
       buttonBox.size,
-      // 1 10px character + 16px * 2 is smaller than the 48px minimum.
-      const Size.square(48.0),
+      // 1 10px character + 16px * 2 is smaller than the default 44px minimum.
+      const Size.square(44.0),
+    );
+  });
+
+  testWidgets('Minimum size parameter', (WidgetTester tester) async {
+    final double minSize = 60.0;
+    await tester.pumpWidget(
+      new Center(child: new CupertinoButton(
+        child: new Text('X', style: testStyle),
+        onPressed: null,
+        minSize: minSize,
+      ))
+    );
+    final RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
+    expect(
+      buttonBox.size,
+      // 1 10px character + 16px * 2 is smaller than defined 60.0px minimum
+      new Size.square(minSize),
     );
   });