Remove window reference from doc snippet (#123176)

Remove window reference from doc snippet

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -42,6 +42,7 @@ import 'object.dart';
 // late Map<Listenable, VoidCallback>? handles;
 // late DiagnosticsTreeStyle style;
 // late IconData icon;
+// late double devicePixelRatio;
 
 /// The various priority levels used to filter which diagnostics are shown and
 /// omitted.
@@ -3218,7 +3219,7 @@ mixin Diagnosticable {
   ///     // without a space.
   ///     properties.add(DoubleProperty(
   ///       'device pixel ratio',
-  ///       ui.window.devicePixelRatio,
+  ///       devicePixelRatio,
   ///       tooltip: 'physical pixels per logical pixel',
   ///     ));
   ///