Fix tapcancel on slop.

Make sure to send tapcancel when the primary pointer fails because of
slop, even if the gesture won by default.

Also, minor cleanup and clarification of an invariant.

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

@@ -146,8 +146,7 @@ class GestureArena {
     } else {
       assert(disposition == GestureDisposition.accepted);
       if (state.isOpen) {
-        if (state.eagerWinner == null)
-          state.eagerWinner = member;
+        state.eagerWinner ??= member;
       } else {
         _resolveInFavorOf(key, state, member);
       }

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

@@ -112,7 +112,7 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
       // TODO(abarth): Maybe factor the slop handling out into a separate class?
       if (event.type == 'pointermove' && _getDistance(event) > kTouchSlop) {
         resolve(GestureDisposition.rejected);
-        stopTrackingPointer(event.pointer);
+        stopTrackingPointer(primaryPointer);
       } else {
         handlePrimaryPointer(event);
       }

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

@@ -43,6 +43,15 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
     }
   }
 
+  void resolve(GestureDisposition disposition) {
+    if (_wonArena && disposition == GestureDisposition.rejected) {
+      if (onTapCancel != null)
+        onTapCancel();
+      _reset();
+    }      
+    super.resolve(disposition);
+  }
+
   void didExceedDeadline() {
     _checkDown();
   }

packages/unit/test/gestures/tap_test.dart

@@ -161,19 +161,28 @@ void main() {
     tap.onTap = () {
       tapRecognized = true;
     };
+    bool tapCanceled = false;
+    tap.onTapCancel = () {
+      tapCanceled = true;
+    };
 
     tap.addPointer(down3);
     GestureArena.instance.close(3);
     expect(tapRecognized, isFalse);
+    expect(tapCanceled, isFalse);
     router.route(down3);
     expect(tapRecognized, isFalse);
+    expect(tapCanceled, isFalse);
 
     router.route(move3);
     expect(tapRecognized, isFalse);
+    expect(tapCanceled, isTrue);
     router.route(up3);
     expect(tapRecognized, isFalse);
+    expect(tapCanceled, isTrue);
     GestureArena.instance.sweep(3);
     expect(tapRecognized, isFalse);
+    expect(tapCanceled, isTrue);
 
     tap.dispose();
   });