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

pkg/compiler/lib/src/serialization/resolved_ast_serialization.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.resolved_ast;
 
 import '../common.dart';
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
@@ skipping 392 matching lines @@
             parse(parser);
           } on ParserError catch (e) {
             reporter.internalError(element, '$e');
           }
           return listener.popNode();
         });
       });
     }
 
     /// Computes the [Node] for the element based on the [AstKind].
+    // ignore: MISSING_RETURN
     Node computeNode(AstKind kind) {
       switch (kind) {
         case AstKind.ENUM_INDEX_FIELD:
           AstBuilder builder = new AstBuilder(element.sourcePosition.begin);
           Identifier identifier = builder.identifier('index');
           VariableDefinitions node = new VariableDefinitions(
               null,
               builder.modifiers(isFinal: true),
               new NodeList.singleton(identifier));
           return node;
@@ skipping 275 matching lines @@
         }
       }
     }
     element.resolvedAst =
         new ParsedResolvedAst(element, root, body, elements, uri);
   }
 }
 
 const Key PARAMETER_NODE = const Key('parameter.node');
 const Key PARAMETER_INITIALIZER = const Key('parameter.initializer');