Add a basic test for DatePicker

Most of this work in this patch is building out the test framework to the point
where we can write this test.

packages/flutter/lib/widgets/date_picker.dart

@@ -136,8 +136,6 @@ class DatePickerHeader extends Component {
     TextStyle dayStyle = headerTheme.display3.copyWith(color: dayColor, height: 1.0, fontSize: 100.0);
     TextStyle monthStyle = headerTheme.headline.copyWith(color: dayColor, height: 1.0);
     TextStyle yearStyle = headerTheme.headline.copyWith(color: yearColor, height: 1.0);
-    DateTime firstDate = new DateTime(1900);
-    DateTime lastDate = new DateTime(2101);
 
     return new Container(
       child: new BlockBody([

packages/unit/test/widget/build_utils.dart

+import 'dart:sky' as sky;
 import 'package:sky/rendering.dart';
 import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
 
 const Size _kTestViewSize = const Size(800.0, 600.0);
 
@@ -30,6 +32,28 @@ class TestApp extends App {
   }
 }
 
+class TestGestureEvent extends sky.GestureEvent {
+  TestGestureEvent({
+    this.type,
+    this.primaryPointer,
+    this.x,
+    this.y,
+    this.dx,
+    this.dy,
+    this.velocityX,
+    this.velocityY
+  });
+
+  String type;
+  int primaryPointer;
+  double x;
+  double y;
+  double dx;
+  double dy;
+  double velocityX;
+  double velocityY;
+}
+
 class WidgetTester {
   WidgetTester() {
     _app = new TestApp();
@@ -49,6 +73,42 @@ class WidgetTester {
     _app.walkChildren(walk);
   }
 
+  Widget findWidget(bool predicate(Widget widget)) {
+    try {
+      walkWidgets((Widget widget) {
+        if (predicate(widget))
+          throw widget;
+      });
+    } catch (e) {
+      if (e is Widget)
+        return e;
+      rethrow;
+    }
+    return null;
+  }
+
+  Text findText(String text) {
+    return findWidget((Widget widget) {
+      return widget is Text && widget.data == text;
+    });
+  }
+
+  Point getCenter(Widget widget) {
+    assert(widget != null);
+    RenderBox box = widget.renderObject as RenderBox;
+    assert(box != null);
+    return box.localToGlobal(box.size.center(Point.origin));
+  }
+
+  void tap(Widget widget) {
+    dispatchEvent(new TestGestureEvent(type: 'gesturetap'), getCenter(widget));
+  }
+
+  void dispatchEvent(sky.Event event, Point position) {
+    HitTestResult result = SkyBinding.instance.hitTest(position);
+    SkyBinding.instance.dispatchEvent(event, result);
+  }
+
   void pumpFrame(WidgetBuilder builder) {
     _app.builder = builder;
     Component.flushBuild();

packages/unit/test/widget/date_picker_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'build_utils.dart';
+
+void main() {
+  test('Can select a day', () {
+    WidgetTester tester = new WidgetTester();
+
+    DateTime currentValue;
+
+    Widget builder() {
+      return new Block([
+        new DatePicker(
+          selectedDate: new DateTime.utc(2015, 6, 9, 7, 12),
+          firstDate: new DateTime.utc(2013),
+          lastDate: new DateTime.utc(2018),
+          onChanged: (DateTime dateTime) {
+            currentValue = dateTime;
+          }
+        )
+      ]);
+    }
+
+    tester.pumpFrame(builder);
+    // TODO(abarth): We shouldn't need to pump a second frame here.
+    tester.pumpFrame(builder);
+
+    expect(currentValue, isNull);
+    tester.tap(tester.findText('30'));
+    expect(currentValue, equals(new DateTime(2013, 1, 30)));
+
+  });
+}