Be more discerning with forward transitions

So you don't fade out when going to a popup menu, for example.

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

@@ -5,8 +5,6 @@
 import 'package:flutter/animation.dart';
 import 'package:flutter/widgets.dart';
 
-import 'colors.dart';
-
 class _MaterialPageTransition extends TransitionWithChild {
   _MaterialPageTransition({
     Key key,
@@ -52,7 +50,8 @@ class MaterialPageRoute<T> extends PageRoute<T> {
   final WidgetBuilder builder;
 
   Duration get transitionDuration => kMaterialPageRouteTransitionDuration;
-  Color get barrierColor => Colors.black54;
+  Color get barrierColor => null;
+  bool canTransitionFrom(TransitionRoute nextRoute) => false;
 
   Widget buildPage(BuildContext context) {
     Widget result = builder(context);

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

@@ -150,7 +150,7 @@ abstract class TransitionRoute<T> extends OverlayRoute<T> {
   final ProxyPerformance forwardPerformance = new ProxyPerformance();
 
   void didPushNext(Route nextRoute) {
-    if (nextRoute is TransitionRoute) {
+    if (nextRoute is TransitionRoute && canTransitionTo(nextRoute) && nextRoute.canTransitionFrom(this)) {
       PerformanceView current = forwardPerformance.masterPerformance;
       if (current != null) {
         if (current is TrainHoppingPerformance) {
@@ -177,6 +177,9 @@ abstract class TransitionRoute<T> extends OverlayRoute<T> {
     super.didPushNext(nextRoute);
   }
 
+  bool canTransitionTo(TransitionRoute nextRoute) => true;
+  bool canTransitionFrom(TransitionRoute nextRoute) => true;
+
   Widget wrapTransition(BuildContext context, Widget child) {
     return buildForwardTransition(
       context,
@@ -434,4 +437,6 @@ abstract class PageRoute<T> extends ModalRoute<T> {
   }) : super(completer: completer, settings: settings);
   bool get opaque => true;
   bool get barrierDismissable => false;
+  bool canTransitionTo(TransitionRoute nextRoute) => nextRoute is PageRoute;
+  bool canTransitionFrom(TransitionRoute nextRoute) => nextRoute is PageRoute;
 }

packages/unit/test/widget/page_forward_transitions_test.dart

@@ -27,6 +27,14 @@ class TestTransition extends TransitionComponent {
   }
 }
 
+class TestRoute<T> extends PageRoute<T> {
+  TestRoute(this.child);
+  final Widget child;
+  Duration get transitionDuration => kMaterialPageRouteTransitionDuration;
+  Color get barrierColor => null;
+  Widget buildPage(BuildContext context) => child;
+}
+
 void main() {
   final Duration kTwoTenthsOfTheTransitionDuration = kMaterialPageRouteTransitionDuration * 0.2;
   final Duration kFourTenthsOfTheTransitionDuration = kMaterialPageRouteTransitionDuration * 0.4;
@@ -57,9 +65,11 @@ void main() {
 
       tester.pumpWidget(
         new MaterialApp(
-          routes: <String, RouteBuilder>{
-            '/': (RouteArguments args) {
-              return new Builder(
+          onGenerateRoute: (NamedRouteSettings settings) {
+            switch (settings.name) {
+              case '/':
+                return new TestRoute(
+                  new Builder(
                     key: insideKey,
                     builder: (BuildContext context) {
                       PageRoute route = ModalRoute.of(context);
@@ -76,11 +86,12 @@ void main() {
                         ),
                       ]);
                     }
+                  )
                 );
-            },
-            '/2': (RouteArguments args) => new Text('E'),
-            '/3': (RouteArguments args) => new Text('F'),
-            '/4': (RouteArguments args) => new Text('G'),
+              case '/2': return new TestRoute(new Text('E'));
+              case '/3': return new TestRoute(new Text('F'));
+              case '/4': return new TestRoute(new Text('G'));
+            }
           }
         )
       );