Detecting just Pan events causes an exception in GestureDetector

Fixes #1320

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

@@ -12,7 +12,9 @@ import 'package:sky/src/gestures/pointer_router.dart';
 export 'package:sky/src/gestures/pointer_router.dart' show PointerRouter;
 
 abstract class GestureRecognizer extends GestureArenaMember {
-  GestureRecognizer({ PointerRouter router }) : _router = router;
+  GestureRecognizer({ PointerRouter router }) : _router = router {
+    assert(_router != null);
+  }
 
   PointerRouter _router;
 

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

@@ -190,7 +190,7 @@ class GestureDetectorState extends State<GestureDetector> {
 
   void _syncScale() {
     if (config.onScaleStart == null && config.onScaleUpdate == null && config.onScaleEnd == null) {
-      _scale = _ensureDisposed(_pan);
+      _scale = _ensureDisposed(_scale);
     } else {
       _ensureScale()
         ..onStart = config.onScaleStart

packages/unit/test/widget/fractionally_sized_box_test.dart

+import 'package:sky/rendering.dart';
 import 'package:sky/widgets.dart';
 import 'package:test/test.dart';
 
@@ -29,7 +30,8 @@ void main() {
         )
       ));
       expect(detectedSize, equals(const Size(50.0, 25.0)));
-      expect(inner.currentContext.findRenderObject().localToGlobal(Point.origin), equals(const Point(25.0, 37.5)));
+      RenderBox box = inner.currentContext.findRenderObject();
+      expect(box.localToGlobal(Point.origin), equals(const Point(25.0, 37.5)));
     });
   });
 }

packages/unit/test/widget/gesture_detector_test.dart

@@ -88,4 +88,38 @@ void main() {
       tester.pumpWidget(new Container());
     });
   });
+
+  test('Pan doesn\'t crash', () {
+    testWidgets((WidgetTester tester) {
+      bool didStartPan = false;
+      Offset panDelta;
+      bool didEndPan = false;
+
+      tester.pumpWidget(
+        new GestureDetector(
+          onPanStart: () {
+            didStartPan = true;
+          },
+          onPanUpdate: (Offset delta) {
+            panDelta = delta;
+          },
+          onPanEnd: (_) {
+            didEndPan = true;
+          },
+          child: new Container()
+        )
+      );
+
+      expect(didStartPan, isFalse);
+      expect(panDelta, isNull);
+      expect(didEndPan, isFalse);
+
+      tester.scrollAt(new Point(10.0, 10.0), new Offset(20.0, 30.0));
+
+      expect(didStartPan, isTrue);
+      expect(panDelta.dx, 20.0);
+      expect(panDelta.dy, 30.0);
+      expect(didEndPan, isTrue);
+    });
+  });
 }

packages/unit/test/widget/widget_tester.dart

@@ -145,7 +145,10 @@ class WidgetTester {
   }
 
   void scroll(Element element, Offset offset, { int pointer: 1 }) {
-    Point startLocation = getCenter(element);
+    scrollAt(getCenter(element), offset, pointer: pointer);
+  }
+
+  void scrollAt(Point startLocation, Offset offset, { int pointer: 1 }) {
     Point endLocation = startLocation + offset;
     TestPointer p = new TestPointer(pointer);
     // Events for the entire press-drag-release gesture are dispatched