use const constructor (#7968)

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

@@ -123,7 +123,7 @@ class CupertinoAlertDialog extends StatelessWidget {
   Widget build(BuildContext context) {
     final List<Widget> children = <Widget>[];
 
-    children.add(new SizedBox(height: 20.0));
+    children.add(const SizedBox(height: 20.0));
 
     if (title != null) {
       children.add(new Padding(
@@ -150,7 +150,7 @@ class CupertinoAlertDialog extends StatelessWidget {
       ));
     }
 
-    children.add(new SizedBox(height: 20.0));
+    children.add(const SizedBox(height: 20.0));
 
     if (actions != null) {
       children.add(new _CupertinoButtonBar(

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

@@ -143,7 +143,7 @@ class _RenderCupertinoSwitch extends RenderConstrainedBox implements SemanticsAc
         _activeColor = activeColor,
         _onChanged = onChanged,
         _vsync = vsync,
-        super(additionalConstraints: new BoxConstraints.tightFor(width: _kSwitchWidth, height: _kSwitchHeight)) {
+        super(additionalConstraints: const BoxConstraints.tightFor(width: _kSwitchWidth, height: _kSwitchHeight)) {
     assert(value != null);
     assert(activeColor != null);
     assert(vsync != null);