Fixed some typos (#4929)

examples/flutter_gallery/lib/demo/overscroll_demo.dart

@@ -31,18 +31,15 @@ class OverscrollDemoState extends State<OverscrollDemo> {
     Completer<Null> completer = new Completer<Null>();
     new Timer(new Duration(seconds: 3), () { completer.complete(null); });
     return completer.future.then((_) {
-      final ScaffoldState scaffoldState = _scaffoldKey.currentState;
-      if (scaffoldState != null) {
-         scaffoldState.showSnackBar(new SnackBar(
-           content: new Text("Refresh complete"),
-           action: new SnackBarAction(
-             label: 'RETRY',
-             onPressed: () {
-               _refreshIndicatorKey.currentState.show();
-             }
-           )
-         ));
-       }
+       _scaffoldKey.currentState?.showSnackBar(new SnackBar(
+         content: new Text("Refresh complete"),
+         action: new SnackBarAction(
+           label: 'RETRY',
+           onPressed: () {
+             _refreshIndicatorKey.currentState.show();
+           }
+         )
+       ));
     });
   }
 

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

@@ -344,7 +344,7 @@ abstract class SchedulerBinding extends BindingBase {
     _postFrameCallbacks.add(callback);
   }
 
-  /// Whether this scheduler as requested that handleBeginFrame be called soon.
+  /// Whether this scheduler has requested that handleBeginFrame be called soon.
   bool get hasScheduledFrame => _hasScheduledFrame;
   bool _hasScheduledFrame = false;
 

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

@@ -157,7 +157,7 @@ class NavigatorObserver {
   /// The [Navigator] pushed the given route.
   void didPush(Route<dynamic> route, Route<dynamic> previousRoute) { }
 
-  /// THe [Navigator] popped the given route.
+  /// The [Navigator] popped the given route.
   void didPop(Route<dynamic> route, Route<dynamic> previousRoute) { }
 }