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

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

 // Copyright (c) 2015, 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.constants;
 
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart'
@@ skipping 348 matching lines @@
 /// Utility class for deserializing [ConstantConstructor]s.
 ///
 /// This is used by the [ConstructorElementZ].
 class ConstantConstructorDeserializer {
   /// Deserializes a [ConstantConstructor] from an [ObjectDecoder].
   ///
   /// The class is called from the [Deserializer] when a constant constructor
   /// 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);
 
     ResolutionDartType readType() {
       return decoder.getType(Key.TYPE);
     }
 
     Map<dynamic /*int|String*/, ConstantExpression> readDefaults() {
       Map<dynamic, ConstantExpression> defaultValues =
@@ skipping 38 matching lines @@
             type, readDefaults(), readFields(), readConstructorInvocation());
       case ConstantConstructorKind.REDIRECTING_GENERATIVE:
         return new RedirectingGenerativeConstantConstructor(
             readDefaults(), readConstructorInvocation());
       case ConstantConstructorKind.REDIRECTING_FACTORY:
         return new RedirectingFactoryConstantConstructor(
             readConstructorInvocation());
     }
   }
 }