Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/js_backend/no_such_method_registry.dart

Index: pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
diff --git a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
index 8c86f466743977d15af879c43e449efd3082abb0..b799a9ac0e882dde5afd377ea1f61d25d6d4832c 100644
--- a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
+++ b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
@@ -4,7 +4,6 @@
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Names, Selectors;
-import '../common_elements.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../types/types.dart';
@@ -92,7 +91,9 @@ class NoSuchMethodRegistry {
   /// that have a return type.
   void onTypeInferenceComplete(GlobalTypeInferenceResults results) {
     otherImpls.forEach((FunctionEntity element) {
-      if (results.resultOfMember(element as MemberEntity).throwsAlways) {
+      if (results.resultOfMember(
+          // ignore: UNNECESSARY_CAST
+          element as MemberEntity).throwsAlways) {
         complexNoReturnImpls.add(element);
       } else {
         complexReturningImpls.add(element);