The dx and dy properties of PointerInputEvents shouldn't be null

They should be 0.0 when there's no offset.

Fixes #1661

packages/flutter/lib/src/gestures/events.dart

@@ -20,28 +20,28 @@ class PointerInputEvent extends InputEvent {
   const PointerInputEvent({
     String type,
     double timeStamp: 0.0,
-    this.pointer,
+    this.pointer: 0,
     this.kind,
-    this.x,
-    this.y,
-    this.dx,
-    this.dy,
-    this.buttons,
-    this.down,
-    this.primary,
-    this.obscured,
-    this.pressure,
-    this.pressureMin,
-    this.pressureMax,
-    this.distance,
-    this.distanceMin,
-    this.distanceMax,
-    this.radiusMajor,
-    this.radiusMinor,
-    this.radiusMin,
-    this.radiusMax,
-    this.orientation,
-    this.tilt
+    this.x: 0.0,
+    this.y: 0.0,
+    this.dx: 0.0,
+    this.dy: 0.0,
+    this.buttons: 0,
+    this.down: false,
+    this.primary: false,
+    this.obscured: false,
+    this.pressure: 0.0,
+    this.pressureMin: 0.0,
+    this.pressureMax: 0.0,
+    this.distance: 0.0,
+    this.distanceMin: 0.0,
+    this.distanceMax: 0.0,
+    this.radiusMajor: 0.0,
+    this.radiusMinor: 0.0,
+    this.radiusMin: 0.0,
+    this.radiusMax: 0.0,
+    this.orientation: 0.0,
+    this.tilt: 0.0
   }) : super(type: type, timeStamp: timeStamp);
 
   final int pointer;

packages/flutter/lib/src/rendering/binding.dart

@@ -62,7 +62,8 @@ class _UiEventConverter {
     Point position = new Point(event.x, event.y);
 
     _PointerState state = _stateForPointer[event.pointer];
-    double dx, dy;
+    double dx = 0.0;
+    double dy = 0.0;
     switch (event.type) {
       case 'pointerdown':
         if (state == null) {