Cancel button behavior for showdatepicker/showtimepicker (#6486)

Cancel button behavior for showdatepicker/showtimepicker

Fixes #6483 

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

@@ -670,8 +670,7 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
 /// Shows a dialog containing a material design date picker.
 ///
 /// The returned [Future] resolves to the date selected by the user when the
-/// user closes the dialog. If the user cancels the dialog, the [Future]
-/// resolves to the initialDate.
+/// user closes the dialog. If the user cancels the dialog, null is returned.
 ///
 /// See also:
 ///
@@ -683,7 +682,7 @@ Future<DateTime> showDatePicker({
   DateTime firstDate,
   DateTime lastDate
 }) async {
-  DateTime picked = await showDialog(
+  return await showDialog(
     context: context,
     child: new _DatePickerDialog(
       initialDate: initialDate,
@@ -691,5 +690,4 @@ Future<DateTime> showDatePicker({
       lastDate: lastDate
     )
   );
-  return picked ?? initialDate;
 }

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

@@ -712,8 +712,7 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
 /// Shows a dialog containing a material design time picker.
 ///
 /// The returned Future resolves to the time selected by the user when the user
-/// closes the dialog. If the user cancels the dialog, the Future resolves to
-/// the [initialTime].
+/// closes the dialog. If the user cancels the dialog, null is returned.
 ///
 /// To show a dialog with [initialTime] equal to the current time:
 /// ```dart
@@ -735,5 +734,5 @@ Future<TimeOfDay> showTimePicker({
   return await showDialog(
     context: context,
     child: new _TimePickerDialog(initialTime: initialTime)
-  ) ?? initialTime;
+  );
 }