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

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

Index: pkg/compiler/lib/src/serialization/constant_serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/constant_serialization.dart b/pkg/compiler/lib/src/serialization/constant_serialization.dart
index 28daa363d6c0d9a582f5989d5bd02a0d9529664f..7ca5f586056122642383b890d4c6c98c86b5921e 100644
--- a/pkg/compiler/lib/src/serialization/constant_serialization.dart
+++ b/pkg/compiler/lib/src/serialization/constant_serialization.dart
@@ -366,6 +366,7 @@ class ConstantConstructorDeserializer {
   /// needs deserialization. The [ObjectDecoder] ensures that any [Element],
   /// [ResolutionDartType], and [ConstantExpression] that the deserialized
   /// [ConstantConstructor] depends upon are available.
+  // ignore: MISSING_RETURN
   static ConstantConstructor deserialize(ObjectDecoder decoder) {
     ConstantConstructorKind kind =
         decoder.getEnum(Key.KIND, ConstantConstructorKind.values);