Add FadeTransition to Android page transition (#9086)

* Add FadeTransition to Android page transition

* Also move starting point up

* Add test

* Use const fractional offset

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

@@ -12,31 +12,38 @@ const double _kMinFlingVelocity = 1.0;  // screen width per second
 
 // Used for Android and Fuchsia.
 class _MountainViewPageTransition extends AnimatedWidget {
-  static final FractionalOffsetTween _kTween = new FractionalOffsetTween(
-    begin: FractionalOffset.bottomLeft,
-    end: FractionalOffset.topLeft
-  );
-
   _MountainViewPageTransition({
     Key key,
-    Animation<double> animation,
+    this.routeAnimation,
     this.child,
   }) : super(
     key: key,
     listenable: _kTween.animate(new CurvedAnimation(
-      parent: animation, // The route's linear 0.0 - 1.0 animation.
+      parent: routeAnimation, // The route's linear 0.0 - 1.0 animation.
       curve: Curves.fastOutSlowIn
     )
   ));
 
+  static final FractionalOffsetTween _kTween = new FractionalOffsetTween(
+    begin: const FractionalOffset(0.0, 0.25),
+    end: FractionalOffset.topLeft
+  );
+
   final Widget child;
+  final Animation<double> routeAnimation;
 
   @override
   Widget build(BuildContext context) {
     // TODO(ianh): tell the transform to be un-transformed for hit testing
     return new SlideTransition(
       position: listenable,
-      child: child
+      child: new FadeTransition(
+        opacity: new CurvedAnimation(
+          parent: routeAnimation,
+          curve: Curves.easeIn, // Eyeballed from other Material apps.
+        ),
+        child: child,
+      ),
     );
   }
 }
@@ -272,7 +279,7 @@ class MaterialPageRoute<T> extends PageRoute<T> {
       );
     } else {
       return new _MountainViewPageTransition(
-        animation: animation,
+        routeAnimation: animation,
         child: child
       );
     }

packages/flutter/test/material/page_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('test Android page transition', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        theme: new ThemeData(platform: TargetPlatform.android),
+        home: new Material(child: new Text('Page 1')),
+        routes: <String, WidgetBuilder>{
+          '/next': (BuildContext context) {
+            return new Material(child: new Text('Page 2'));
+          },
+        }
+      )
+    );
+
+    final Point widget1TopLeft = tester.getTopLeft(find.text('Page 1'));
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 1));
+
+    final Opacity widget2Opacity =
+        tester.element(find.text('Page 2')).ancestorWidgetOfExactType(Opacity);
+    final Point widget2TopLeft = tester.getTopLeft(find.text('Page 2'));
+    final Size widget2Size = tester.getSize(find.text('Page 2'));
+
+    expect(widget1TopLeft.x == widget2TopLeft.x, true);
+    // Page 1 is above page 2 mid-transition.
+    expect(widget1TopLeft.y < widget2TopLeft.y, true);
+    // Animation begins 3/4 of the way up the page.
+    expect(widget2TopLeft.y < widget2Size.height / 4.0, true);
+    // Animation starts with page 2 being near transparent.
+    expect(widget2Opacity.opacity < 0.01, true);
+  });
+}