Implement a pushRoute service to satisfy the FlutterView.pushRoute API (#10415)

Fixes https://github.com/flutter/flutter/issues/10399

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

@@ -166,6 +166,15 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
     return await navigator.maybePop();
   }
 
+  @override
+  Future<bool> didPushRoute(String route) async {
+    assert(mounted);
+    final NavigatorState navigator = _navigator.currentState;
+    assert(navigator != null);
+    navigator.pushNamed(route);
+    return true;
+  }
+
   @override
   void didChangeMetrics() {
     setState(() {

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

@@ -37,6 +37,14 @@ abstract class WidgetsBindingObserver {
   /// its current route if possible.
   Future<bool> didPopRoute() => new Future<bool>.value(false);
 
+  /// Called when the host tells the app to push a new route onto the
+  /// navigator.
+  ///
+  /// Observers are expected to return true if they were able to
+  /// handle the notification.  Observers are notified in registration
+  /// order until one returns true.
+  Future<bool> didPushRoute(String route) => new Future<bool>.value(false);
+
   /// Called when the application's dimensions change. For example,
   /// when a phone is rotated.
   void didChangeMetrics() { }
@@ -198,10 +206,23 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
     SystemNavigator.pop();
   }
 
-  Future<dynamic> _handleNavigationInvocation(MethodCall methodCall) async {
-    if (methodCall.method == 'popRoute')
-      handlePopRoute();
-    // TODO(abarth): Handle 'pushRoute'.
+  /// Called when the host tells the app to push a new route onto the
+  /// navigator.
+  Future<Null> handlePushRoute(String route) async {
+    for (WidgetsBindingObserver observer in new List<WidgetsBindingObserver>.from(_observers)) {
+      if (await observer.didPushRoute(route))
+        return;
+    }
+  }
+
+  Future<dynamic> _handleNavigationInvocation(MethodCall methodCall) {
+    switch (methodCall.method) {
+      case 'popRoute':
+        return handlePopRoute();
+      case 'pushRoute':
+        return handlePushRoute(methodCall.arguments);
+    }
+    return new Future<Null>.value();
   }
 
   /// Called when the application lifecycle state changes.

packages/flutter/test/widgets/binding_test.dart

@@ -26,6 +26,16 @@ class AppLifecycleStateObserver extends WidgetsBindingObserver {
   }
 }
 
+class PushRouteObserver extends WidgetsBindingObserver {
+  String pushedRoute;
+
+  @override
+  Future<bool> didPushRoute(String route) async {
+    pushedRoute = route;
+    return true;
+  }
+}
+
 void main() {
   setUp(() {
     WidgetsFlutterBinding.ensureInitialized();
@@ -61,4 +71,17 @@ void main() {
     await BinaryMessages.handlePlatformMessage('flutter/lifecycle', message, (_) {});
     expect(observer.lifecycleState, AppLifecycleState.suspending);
   });
+
+  testWidgets('didPushRoute callback', (WidgetTester tester) async {
+    final PushRouteObserver observer = new PushRouteObserver();
+    WidgetsBinding.instance.addObserver(observer);
+
+    final String testRouteName = 'testRouteName';
+    final ByteData message = const JSONMethodCodec().encodeMethodCall(
+      new MethodCall('pushRoute', testRouteName));
+    await BinaryMessages.handlePlatformMessage('flutter/navigation', message, (_) {});
+    expect(observer.pushedRoute, testRouteName);
+
+    WidgetsBinding.instance.removeObserver(observer);
+  });
 }