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

pkg/compiler/lib/src/serialization/type_serialization.dart

Index: pkg/compiler/lib/src/serialization/type_serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/type_serialization.dart b/pkg/compiler/lib/src/serialization/type_serialization.dart
index c5f68c848f3fdf76fde99bba3fb3c6e9929190fc..89ab51055155832c8f7924e3baa928508f9a2da6 100644
--- a/pkg/compiler/lib/src/serialization/type_serialization.dart
+++ b/pkg/compiler/lib/src/serialization/type_serialization.dart
@@ -68,6 +68,7 @@ class TypeDeserializer {
   /// needs deserialization. The [ObjectDecoder] ensures that any [Element],
   /// other [ResolutionDartType] that the deserialized [ResolutionDartType]
   /// depends upon are available.
+  // ignore: MISSING_RETURN
   static ResolutionDartType deserialize(ObjectDecoder decoder) {
     ResolutionTypeKind typeKind =
         decoder.getEnum(Key.KIND, ResolutionTypeKind.values);