Tap.dart: Fixes the spacing to the right side of reason (#43246)

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

@@ -497,7 +497,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   @protected
   @override
   void handleTapCancel({PointerDownEvent down, PointerCancelEvent cancel, String reason}) {
-    final String note = reason == '' ? reason : ' $reason';
+    final String note = reason == '' ? reason : '$reason ';
     switch (down.buttons) {
       case kPrimaryButton:
         if (onTapCancel != null)

packages/flutter/test/gestures/tap_test.dart

@@ -380,6 +380,32 @@ void main() {
     tap.dispose();
   });
 
+  testGesture('onTapCancel should show reason in the proper format', (GestureTester tester) {
+    final TapGestureRecognizer tap = TapGestureRecognizer();
+
+    tap.onTapCancel = () {
+      throw Exception(test);
+    };
+
+    final FlutterExceptionHandler previousErrorHandler = FlutterError.onError;
+    bool gotError = false;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      expect(details.toString().contains('"spontaneous onTapCancel"') , isTrue);
+      gotError = true;
+    };
+
+    const int pointer = 1;
+    tap.addPointer(const PointerDownEvent(pointer: pointer));
+    tester.closeArena(pointer);
+    tester.async.elapse(const Duration(milliseconds: 500));
+    tester.route(const PointerCancelEvent(pointer: pointer));
+
+    expect(gotError, isTrue);
+
+    FlutterError.onError = previousErrorHandler;
+    tap.dispose();
+  });
+
   testGesture('No duplicate tap events', (GestureTester tester) {
     final TapGestureRecognizer tapA = TapGestureRecognizer();
     final TapGestureRecognizer tapB = TapGestureRecognizer();