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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.serialization.util;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../diagnostics/messages.dart';
@@ skipping 173 matching lines @@
     case SendStructureKind.DEFERRED_PREFIX:
       DeferredPrefixStructure structure = sendStructure;
       encoder.setElement(Key.PREFIX, structure.prefix);
       serializeSendStructure(
           structure.sendStructure, encoder.createObject(Key.SEND_STRUCTURE));
       break;
   }
 }
 
 /// Deserialize a [SendStructure] from [decoder].
+// ignore: MISSING_RETURN
 SendStructure deserializeSendStructure(ObjectDecoder decoder) {
   SendStructureKind kind = decoder.getEnum(Key.KIND, SendStructureKind.values);
   switch (kind) {
     case SendStructureKind.IF_NULL:
       return const IfNullStructure();
     case SendStructureKind.LOGICAL_AND:
       return const LogicalAndStructure();
     case SendStructureKind.LOGICAL_OR:
       return const LogicalOrStructure();
     case SendStructureKind.IS:
@@ skipping 130 matching lines @@
       encoder.setEnum(Key.SUB_KIND, structure.constantInvokeKind);
       encoder.setConstant(Key.CONSTANT, structure.constant);
       break;
     case NewStructureKind.LATE_CONST:
       throw new UnsupportedError(
           'Unsupported NewStructure kind ${newStructure.kind}.');
   }
 }
 
 /// Deserialize a [NewStructure] from [decoder].
+// ignore: MISSING_RETURN
 NewStructure deserializeNewStructure(ObjectDecoder decoder) {
   NewStructureKind kind = decoder.getEnum(Key.KIND, NewStructureKind.values);
   switch (kind) {
     case NewStructureKind.NEW_INVOKE:
       ConstructorAccessKind constructorAccessKind =
           decoder.getEnum(Key.SUB_KIND, ConstructorAccessKind.values);
       Element element = decoder.getElement(Key.ELEMENT);
       ResolutionDartType type = decoder.getType(Key.TYPE);
       ConstructorAccessSemantics semantics =
           new ConstructorAccessSemantics(constructorAccessKind, element, type);
@@ skipping 60 matching lines @@
       encoder.setEnum(Key.SUB_KIND, compoundAccess.compoundAccessKind);
       encoder.setElement(Key.GETTER, semantics.getter);
       encoder.setElement(Key.SETTER, semantics.setter);
       break;
     case AccessKind.CONSTANT:
       throw new UnsupportedError('Unsupported access kind: ${semantics.kind}');
   }
 }
 
 /// Deserialize a [AccessSemantics] from [decoder].
+// ignore: MISSING_RETURN
 AccessSemantics deserializeAccessSemantics(ObjectDecoder decoder) {
   AccessKind kind = decoder.getEnum(Key.KIND, AccessKind.values);
   switch (kind) {
     case AccessKind.EXPRESSION:
       return const DynamicAccess.expression();
     case AccessKind.THIS:
       return const DynamicAccess.thisAccess();
     case AccessKind.THIS_PROPERTY:
       return new DynamicAccess.thisProperty(deserializeName(decoder));
     case AccessKind.DYNAMIC_PROPERTY:
@@ skipping 145 matching lines @@
   ObjectDecoder sourceSpanDecoder =
       object.getObject(Key.SOURCE_SPAN, isOptional: true);
   if (sourceSpanDecoder != null) {
     sourceSpan = deserializeSourceSpan(sourceSpanDecoder);
   }
   MessageKind messageKind = object.getEnum(Key.KIND, MessageKind.values);
   Map<String, dynamic> messageArguments =
       deserializeMessageArguments(object, Key.ARGUMENTS);
   return new WrappedMessage(sourceSpan, messageKind, messageArguments);
 }