Let MaterialApp override the initial route (#6216)

Fixes #6215

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

@@ -52,6 +52,7 @@ class MaterialApp extends StatefulWidget {
     this.theme,
     this.home,
     this.routes: const <String, WidgetBuilder>{},
+    this.initialRoute,
     this.onGenerateRoute,
     this.onLocaleChanged,
     this.debugShowMaterialGrid: false,
@@ -110,6 +111,11 @@ class MaterialApp extends StatefulWidget {
   /// build the page instead.
   final Map<String, WidgetBuilder> routes;
 
+  /// The name of the first route to show.
+  ///
+  /// Defaults to [Window.defaultRouteName].
+  final String initialRoute;
+
   /// The route generator callback used when the app is navigated to a
   /// named route.
   final RouteFactory onGenerateRoute;
@@ -254,6 +260,7 @@ class _MaterialAppState extends State<MaterialApp> {
         // blue[500] is the primary color of the default theme
         color: config.color ?? theme?.primaryColor ?? Colors.blue[500],
         navigatorObserver: _heroController,
+        initialRoute: config.initialRoute,
         onGenerateRoute: _onGenerateRoute,
         onLocaleChanged: config.onLocaleChanged,
         showPerformanceOverlay: config.showPerformanceOverlay,

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

@@ -43,6 +43,7 @@ class WidgetsApp extends StatefulWidget {
     this.textStyle,
     this.color,
     this.navigatorObserver,
+    this.initialRoute,
     this.onLocaleChanged,
     this.showPerformanceOverlay: false,
     this.showSemanticsDebugger: false,
@@ -70,6 +71,11 @@ class WidgetsApp extends StatefulWidget {
   /// named route.
   final RouteFactory onGenerateRoute;
 
+  /// The name of the first route to show.
+  ///
+  /// Defaults to [Window.defaultRouteName].
+  final String initialRoute;
+
   /// Callback that is called when the operating system changes the
   /// current locale.
   final LocaleChangedCallback onLocaleChanged;
@@ -176,7 +182,7 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
           color: config.color,
           child: new Navigator(
             key: _navigator,
-            initialRoute: ui.window.defaultRouteName,
+            initialRoute: config.initialRoute ?? ui.window.defaultRouteName,
             onGenerateRoute: config.onGenerateRoute,
             observer: config.navigatorObserver
           )