Fix asserts in Draggable dismounting (#4644)

In writing a test for #1927, I found a number of bugs in how Draggable shuts
down. Previously it would leak its recongizer. Now it disposes its recognizer
and the recognizer knows how to be disposed cleanly.

Fixes #1927

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

@@ -5,6 +5,8 @@
 import 'dart:async';
 import 'dart:ui' show Point, Offset;
 
+import 'package:meta/meta.dart';
+
 import 'arena.dart';
 import 'binding.dart';
 import 'constants.dart';
@@ -147,7 +149,9 @@ abstract class MultiDragPointerState {
   }
 
   /// Releases any resources used by the object.
+  @mustCallSuper
   void dispose() {
+    _arenaEntry?.resolve(GestureDisposition.rejected);
     assert(() { _pendingDelta = null; return true; });
   }
 }
@@ -258,14 +262,14 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
     assert(_pointers != null);
     assert(_pointers.containsKey(pointer));
     GestureBinding.instance.pointerRouter.removeRoute(pointer, _handleEvent);
-    _pointers[pointer].dispose();
+    _pointers.remove(pointer).dispose();
-    _pointers.remove(pointer);
   }
 
   @override
   void dispose() {
-    for (int pointer in _pointers.keys)
+    for (int pointer in _pointers.keys.toList())
       _removeState(pointer);
+    assert(_pointers.isEmpty);
     _pointers = null;
     super.dispose();
   }

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

@@ -262,6 +262,12 @@ class _DraggableState<T> extends State<DraggableBase<T>> {
     _recognizer = config.createRecognizer(_startDrag);
   }
 
+  @override
+  void dispose() {
+    _recognizer.dispose();
+    super.dispose();
+  }
+
   GestureRecognizer _recognizer;
   int _activeCount = 0;
 

packages/flutter/test/widget/draggable_test.dart

@@ -871,6 +871,75 @@ void main() {
       await tester.pump();
     }
   });
+
+  testWidgets('Draggable disposes recognizer', (WidgetTester tester) async {
+    bool didTap = false;
+    await tester.pumpWidget(new Overlay(
+      initialEntries: <OverlayEntry>[
+        new OverlayEntry(
+          builder: (BuildContext context) => new GestureDetector(
+            onTap: () {
+              didTap = true;
+            },
+            child: new Draggable<dynamic>(
+              child: new Container(
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFFFFFF00)
+                )
+              ),
+              feedback: new Container(
+                width: 100.0,
+                height: 100.0,
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFFFF0000)
+                )
+              )
+            )
+          )
+        )
+      ]
+    ));
+
+    await tester.startGesture(const Point(10.0, 10.0));
+    expect(didTap, isFalse);
+
+    // This tears down the draggable without terminating the gesture sequence,
+    // which used to trigger asserts in the multi-drag gesture recognizer.
+    await tester.pumpWidget(new Container(key: new UniqueKey()));
+    expect(didTap, isFalse);
+  });
+
+  testWidgets('Draggable plays nice with onTap', (WidgetTester tester) async {
+    await tester.pumpWidget(new Overlay(
+      initialEntries: <OverlayEntry>[
+        new OverlayEntry(
+          builder: (BuildContext context) => new GestureDetector(
+            onTap: () { /* registers a tap recognizer */ },
+            child: new Draggable<dynamic>(
+              child: new Container(
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFFFFFF00)
+                )
+              ),
+              feedback: new Container(
+                width: 100.0,
+                height: 100.0,
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFFFF0000)
+                )
+              )
+            )
+          )
+        )
+      ]
+    ));
+
+    TestGesture firstGesture = await tester.startGesture(const Point(10.0, 10.0), pointer: 24);
+    TestGesture secondGesture = await tester.startGesture(const Point(10.0, 20.0), pointer: 25);
+
+    await firstGesture.moveBy(new Offset(100.0, 0.0));
+    await secondGesture.up();
+  });
 }
 
 class DragTargetData { }