Remove PopupMenu margin

The margins make using showMenu's menuPosition argument difficult and they're not really needed.

I also made a few small gratuitous changes in navigator.dart.

examples/stocks/lib/stock_menu.dart

@@ -6,12 +6,14 @@ part of stocks;
 
 enum _MenuItems { autorefresh, autorefreshCheckbox, add, remove }
 
+const double _kMenuMargin = 16.0; // 24.0 on tablet
+
 Future showStockMenu(NavigatorState navigator, { bool autorefresh, ValueChanged onAutorefreshChanged }) async {
   switch (await showMenu(
     navigator: navigator,
     position: new MenuPosition(
-      right: sky.view.paddingRight,
-      top: sky.view.paddingTop
+      right: sky.view.paddingRight + _kMenuMargin,
+      top: sky.view.paddingTop + _kMenuMargin
     ),
     builder: (NavigatorState navigator) {
       return <PopupMenuItem>[

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

@@ -63,16 +63,11 @@ class NavigatorState extends State<Navigator> {
   }
 
   void pushNamed(String name) {
-    RouteBuilder builder;
-    if (!config.routes.containsKey(name)) {
+    RouteBuilder generateRoute() {
       assert(config.onGenerateRoute != null);
-      builder = config.onGenerateRoute(name);
-    } else {
-      builder = config.routes[name];
+      return config.onGenerateRoute(name);
     }
-    if (builder == null)
-      builder = config.onUnknownRoute; // 404!
-    assert(builder != null); // 404 getting your 404!
+    RouteBuilder builder = config.routes[name] ?? generateRoute() ?? config.onUnknownRoute;
     push(new PageRoute(builder));
   }
 
@@ -87,7 +82,6 @@ class NavigatorState extends State<Navigator> {
     assert(!_debugCurrentlyHaveRoute(route));
     setState(() {
       while (currentRoute.ephemeral) {
-        assert(currentRoute.ephemeral);
         currentRoute.didPop(null);
         _currentPosition -= 1;
       }

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

@@ -21,7 +21,6 @@ import 'package:sky/src/widgets/transitions.dart';
 const Duration _kMenuDuration = const Duration(milliseconds: 300);
 const double _kMenuCloseIntervalEnd = 2.0 / 3.0;
 const double _kMenuWidthStep = 56.0;
-const double _kMenuMargin = 16.0; // 24.0 on tablet
 const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
 const double _kMenuMaxWidth = 5.0 * _kMenuWidthStep;
 const double _kMenuHorizontalPadding = 16.0;
@@ -75,8 +74,6 @@ class PopupMenu extends StatelessComponent {
     return new FadeTransition(
       performance: performance,
       opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: new Interval(0.0, 1.0 / 3.0)),
-      child: new Container(
-        margin: new EdgeDims.all(_kMenuMargin),
       child: new BuilderTransition(
         performance: performance,
         variables: [width, height],
@@ -108,7 +105,6 @@ class PopupMenu extends StatelessComponent {
           );
         }
       )
-      )
     );
   }
 }