Add device pixel ratio to MediaQuery

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

@@ -77,15 +77,11 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
 
   GlobalObjectKey _navigator;
 
-  Size _size;
-  EdgeDims _padding;
   LocaleQueryData _localeData;
 
   void initState() {
     super.initState();
     _navigator = new GlobalObjectKey(this);
-    _size = ui.window.size;
-    _padding = _getPadding(ui.window);
     didChangeLocale(ui.window.locale);
     WidgetFlutterBinding.instance.addObserver(this);
   }
@@ -106,10 +102,10 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
     return result;
   }
 
-  void didChangeSize(Size size) {
+  void didChangeMetrics() {
     setState(() {
-      _size = size;
-      _padding = _getPadding(ui.window);
+      // The properties of ui.window have changed. We use them in our build
+      // function, so we need setState(), but we don't cache anything locally.
     });
   }
 
@@ -150,7 +146,11 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
 
     ThemeData theme = config.theme ?? new ThemeData.fallback();
     Widget result = new MediaQuery(
-      data: new MediaQueryData(size: _size, padding: _padding),
+      data: new MediaQueryData(
+        size: ui.window.size,
+        devicePixelRatio: ui.window.devicePixelRatio,
+        padding: _getPadding(ui.window)
+      ),
       child: new LocaleQuery(
         data: _localeData,
         child: new AnimatedTheme(

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

@@ -14,7 +14,7 @@ import 'framework.dart';
 
 class BindingObserver {
   bool didPopRoute() => false;
-  void didChangeSize(Size size) { }
+  void didChangeMetrics() { }
   void didChangeLocale(ui.Locale locale) { }
   void didChangeAppLifecycleState(ui.AppLifecycleState state) { }
 }
@@ -62,7 +62,7 @@ class WidgetFlutterBinding extends BindingBase with Scheduler, Gesturer, Rendere
   void handleMetricsChanged() {
     super.handleMetricsChanged();
     for (BindingObserver observer in _observers)
-      observer.didChangeSize(ui.window.size);
+      observer.didChangeMetrics(ui.window.size);
   }
 
   void handleLocaleChanged() {

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

@@ -16,11 +16,16 @@ enum Orientation {
 
 /// The result of a media query.
 class MediaQueryData {
-  const MediaQueryData({ this.size, this.padding });
+  const MediaQueryData({ this.size, this.devicePixelRatio, this.padding });
 
   /// The size of the media (e.g, the size of the screen).
   final Size size;
 
+  /// The number of device pixels for each logical pixel. This number might not
+  /// be a power of two. Indeed, it might not even be an integer. For example,
+  /// the Nexus 6 has a device pixel ratio of 3.5.
+  final double devicePixelRatio;
+
   /// The padding around the edges of the media (e.g., the screen).
   final EdgeDims padding;