More fixes for the prefer_const_literals_to_create_immutables analyzer check (#15363)

See https://github.com/flutter/flutter/commit/3961306badd313a3f34c5c25a754c1e53a581caf

packages/flutter/test/material/bottom_navigation_bar_test.dart

@@ -553,7 +553,7 @@ Widget boilerplate({ Widget bottomNavigationBar, @required TextDirection textDir
   assert(textDirection != null);
   return new Localizations(
     locale: const Locale('en', 'US'),
-    delegates: <LocalizationsDelegate<dynamic>>[
+    delegates: const <LocalizationsDelegate<dynamic>>[
       DefaultMaterialLocalizations.delegate,
       DefaultWidgetsLocalizations.delegate,
     ],

packages/flutter/test/widgets/wrap_test.dart

@@ -835,7 +835,7 @@ void main() {
       textDirection: TextDirection.ltr,
       spacing: 10.0,
       runSpacing: 10.0,
-      children: <Widget>[
+      children: const <Widget>[
         const SizedBox(width: 200.0, height: 10.0),
         const SizedBox(width: 200.0, height: 10.0),
         const SizedBox(width: 200.0, height: 10.0),
@@ -858,7 +858,7 @@ void main() {
       textDirection: TextDirection.ltr,
       spacing: 10.0,
       runSpacing: 10.0,
-      children: <Widget>[
+      children: const <Widget>[
         const SizedBox(width: 800.0, height: 0.0),
       ],
     ));