Let iOS back gesture listen to edge swipes only (#11559)

* Make iOS back gesture triggerable from the edge only.

* change to positioned

packages/flutter/lib/src/cupertino/page.dart

@@ -450,10 +450,15 @@ class _CupertinoBackGestureDetectorState extends State<_CupertinoBackGestureDete
       fit: StackFit.passthrough,
       children: <Widget>[
         widget.child,
-        new Listener(
-          onPointerDown: _handlePointerDown,
-          behavior: HitTestBehavior.translucent,
-          child: new SizedBox(width: _kBackGestureWidth),
+        new Positioned(
+          left: 0.0,
+          width: _kBackGestureWidth,
+          top: 0.0,
+          bottom: 0.0,
+          child: new Listener(
+            onPointerDown: _handlePointerDown,
+            behavior: HitTestBehavior.translucent,
+          ),
         ),
       ],
     );

packages/flutter/test/cupertino/page_test.dart

@@ -141,4 +141,48 @@ void main() {
     // Page 1 is back where it started.
     expect(widget1InitialTopLeft == widget1TransientTopLeft, true);
   });
-}
+
+  testWidgets('test only edge swipes work', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          return new CupertinoPageRoute<Null>(
+            settings: settings,
+            builder: (BuildContext context) {
+              final String pageNumber = settings.name == '/' ? "1" : "2";
+              return new Center(child: new Text('Page $pageNumber'));
+            }
+          );
+        },
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400));
+
+    // Page 2 covers page 1.
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    // Drag from the middle to the right.
+    TestGesture gesture = await tester.startGesture(const Offset(200.0, 200.0));
+    await gesture.moveBy(const Offset(300.0, 0.0));
+    await tester.pump();
+
+    // Nothing should happen.
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    // Now drag from the edge.
+    gesture = await tester.startGesture(const Offset(5.0, 200.0));
+    await gesture.moveBy(const Offset(300.0, 0.0));
+    await tester.pump();
+
+    // Page 1 is now visible.
+    expect(find.text('Page 1'), isOnstage);
+    expect(find.text('Page 2'), isOnstage);
+  });
+}