Use a lookup table to calculate the number of days in a month (#10860)

Fixes https://github.com/flutter/flutter/issues/10541

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

@@ -220,14 +220,24 @@ class DayPicker extends StatelessWidget {
     }).toList(growable: false);
   }
 
+  // Do not use this directly - call getDaysInMonth instead.
+  static const List<int> _kDaysInMonth = const <int>[31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+  static int getDaysInMonth(int year, int month) {
+    if (month == DateTime.FEBRUARY) {
+      final bool isLeapYear = (year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0);
+      if (isLeapYear)
+        return 29;
+    }
+    return _kDaysInMonth[month - 1];
+  }
+
   @override
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context);
     final int year = displayedMonth.year;
     final int month = displayedMonth.month;
-    // Dart's Date time constructor is very forgiving and will understand
-    // month 13 as January of the next year. :)
-    final int daysInMonth = new DateTime(year, month + 1).difference(new DateTime(year, month)).inDays;
+    final int daysInMonth = getDaysInMonth(year, month);
     // This assumes a start day of SUNDAY, but could be changed.
     final int firstWeekday = new DateTime(year, month).weekday % 7;
     final List<Widget> labels = <Widget>[];

packages/flutter/test/material/date_picker_test.dart

@@ -329,4 +329,13 @@ void main() {
       });
     });
   });
+
+  test('days in month', () {
+    expect(DayPicker.getDaysInMonth(2017, 10), 31);
+    expect(DayPicker.getDaysInMonth(2017, 6), 30);
+    expect(DayPicker.getDaysInMonth(2017, 2), 28);
+    expect(DayPicker.getDaysInMonth(2016, 2), 29);
+    expect(DayPicker.getDaysInMonth(2000, 2), 29);
+    expect(DayPicker.getDaysInMonth(1900, 2), 28);
+  });
 }