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

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

Index: pkg/compiler/lib/src/serialization/serialization_util.dart
diff --git a/pkg/compiler/lib/src/serialization/serialization_util.dart b/pkg/compiler/lib/src/serialization/serialization_util.dart
index 8816cbdd5e24167abdefb4592ee6af193d56fca3..b58d339fbe4154f259f078cf9dcd1e41f217fa81 100644
--- a/pkg/compiler/lib/src/serialization/serialization_util.dart
+++ b/pkg/compiler/lib/src/serialization/serialization_util.dart
@@ -191,6 +191,7 @@ void serializeSendStructure(
 }
 
 /// Deserialize a [SendStructure] from [decoder].
+// ignore: MISSING_RETURN
 SendStructure deserializeSendStructure(ObjectDecoder decoder) {
   SendStructureKind kind = decoder.getEnum(Key.KIND, SendStructureKind.values);
   switch (kind) {
@@ -341,6 +342,7 @@ void serializeNewStructure(NewStructure newStructure, ObjectEncoder encoder) {
 }
 
 /// Deserialize a [NewStructure] from [decoder].
+// ignore: MISSING_RETURN
 NewStructure deserializeNewStructure(ObjectDecoder decoder) {
   NewStructureKind kind = decoder.getEnum(Key.KIND, NewStructureKind.values);
   switch (kind) {
@@ -421,6 +423,7 @@ void serializeAccessSemantics(
 }
 
 /// Deserialize a [AccessSemantics] from [decoder].
+// ignore: MISSING_RETURN
 AccessSemantics deserializeAccessSemantics(ObjectDecoder decoder) {
   AccessKind kind = decoder.getEnum(Key.KIND, AccessKind.values);
   switch (kind) {