Add the position at which drags start

Fixes #1807

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

@@ -15,11 +15,11 @@ enum DragState {
   accepted
 }
 
-typedef void GestureDragStartCallback();
+typedef void GestureDragStartCallback(ui.Point globalPosition);
 typedef void GestureDragUpdateCallback(double delta);
 typedef void GestureDragEndCallback(ui.Offset velocity);
 
-typedef void GesturePanStartCallback();
+typedef void GesturePanStartCallback(ui.Point globalPosition);
 typedef void GesturePanUpdateCallback(ui.Offset delta);
 typedef void GesturePanEndCallback(ui.Offset velocity);
 
@@ -43,6 +43,7 @@ abstract class _DragGestureRecognizer<T extends dynamic> extends GestureRecogniz
   GestureDragEndCallback onEnd;
 
   DragState _state = DragState.ready;
+  ui.Point _initialPosition;
   T _pendingDragDelta;
 
   T get _initialPendingDragDelta;
@@ -56,6 +57,7 @@ abstract class _DragGestureRecognizer<T extends dynamic> extends GestureRecogniz
     _velocityTrackers[event.pointer] = new ui.VelocityTracker();
     if (_state == DragState.ready) {
       _state = DragState.possible;
+      _initialPosition = event.position;
       _pendingDragDelta = _initialPendingDragDelta;
     }
   }
@@ -85,7 +87,7 @@ abstract class _DragGestureRecognizer<T extends dynamic> extends GestureRecogniz
       T delta = _pendingDragDelta;
       _pendingDragDelta = _initialPendingDragDelta;
       if (onStart != null)
-        onStart();
+        onStart(_initialPosition);
       if (delta != _initialPendingDragDelta && onUpdate != null)
         onUpdate(delta);
     }

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

@@ -49,7 +49,7 @@ class _Drawer extends StatelessComponent {
 
   Widget build(BuildContext context) {
     return new GestureDetector(
-      onHorizontalDragStart: () {
+      onHorizontalDragStart: (_) {
         if (interactive)
           route._takeControl();
       },

packages/flutter/lib/src/widgets/dismissable.dart

@@ -115,7 +115,7 @@ class _DismissableState extends State<Dismissable> {
       _maybeCallOnResized();
   }
 
-  void _handleDragStart() {
+  void _handleDragStart(_) {
     if (_fadePerformance.isAnimating)
       return;
     setState(() {

packages/flutter/lib/src/widgets/scrollable.dart

@@ -239,7 +239,7 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     _animation.stop();
   }
 
-  void _handleDragStart() {
+  void _handleDragStart(_) {
     scheduleMicrotask(dispatchOnScrollStart);
   }
 

packages/unit/test/gestures/scroll_test.dart

@@ -12,7 +12,7 @@ void main() {
     TapGestureRecognizer tap = new TapGestureRecognizer(router: router);
 
     bool didStartPan = false;
-    pan.onStart = () {
+    pan.onStart = (_) {
       didStartPan = true;
     };
 

packages/unit/test/widget/gesture_detector_test.dart

@@ -14,7 +14,7 @@ void main() {
       bool didEndDrag = false;
 
       Widget widget = new GestureDetector(
-        onVerticalDragStart: () {
+        onVerticalDragStart: (_) {
           didStartDrag = true;
         },
         onVerticalDragUpdate: (double scrollDelta) {
@@ -97,7 +97,7 @@ void main() {
 
       tester.pumpWidget(
         new GestureDetector(
-          onPanStart: () {
+          onPanStart: (_) {
             didStartPan = true;
           },
           onPanUpdate: (Offset delta) {