Fix minor misspellings in comments. (#5641)

examples/flutter_gallery/test/pesto_test.dart

@@ -55,7 +55,7 @@ void main() {
   // Regression test for https://github.com/flutter/flutter/pull/5168
   testWidgets('Pesto appbar heroics', (WidgetTester tester) async {
     await tester.pumpWidget(
-      // The bug only manifests itself when the screen's orientation is portait
+      // The bug only manifests itself when the screen's orientation is portrait
       new Center(
         child: new SizedBox(
           width: 400.0,

examples/hello_services/lib/main.dart

@@ -52,7 +52,7 @@ class _HelloServicesState extends State<HelloServices> {
     final Map<String, dynamic> reply = await HostMessages.sendJSON('getLocation', message);
     // If the widget was removed from the tree while the message was in flight,
     // we want to discard the reply rather than calling setState to update our
-    // non-existant appearance.
+    // non-existent appearance.
     if (!mounted)
       return;
     setState(() {