Incorporate compat info changes into flutter engine

Changes related to compatibility info require changes in
Flutter engine libraries. This CL adds null-asserts wherever
necessary to maintain behavior and adds some small modifications
to handle the change in nullability. Warnings about unnecessary
null assertions are disabled temporarily in this CL as well.

Original issue: dart-lang/sdk#41905

lib/web_ui/analysis_options.yaml

@@ -16,8 +16,9 @@ analyzer:
 
 linter:
   rules:
-    - always_declare_return_types
-    - always_put_control_body_on_new_line
+    always_declare_return_types: true
+    always_put_control_body_on_new_line: true
+    unnecessary_non_null_assertion: false
     # TODO(uncomment) - always_specify_types
     # TODO(uncomment) - annotate_overrides
     # TODO(uncomment) - avoid_classes_with_only_static_members

lib/web_ui/lib/src/engine/window.dart

@@ -280,15 +280,15 @@ class EngineWindow extends ui.Window {
 
   static List<ui.Locale> parseBrowserLanguages() {
     // TODO(yjbanov): find a solution for IE
-    final bool languagesFeatureMissing = !js_util.hasProperty(html.window.navigator, 'languages');
-    if (languagesFeatureMissing || html.window.navigator.languages!.isEmpty) {
+    var languages = html.window.navigator.languages;
+    if (languages == null || languages.isEmpty) {
       // To make it easier for the app code, let's not leave the locales list
       // empty. This way there's fewer corner cases for apps to handle.
       return const [_defaultLocale];
     }
 
     final List<ui.Locale> locales = <ui.Locale>[];
-    for (final String language in html.window.navigator.languages!) {
+    for (final String language in languages) {
       final List<String> parts = language.split('-');
       if (parts.length > 1) {
         locales.add(ui.Locale(parts.first, parts.last));