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

pkg/compiler/lib/src/resolution/send_resolver.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.semantics_visitor.resolver;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
@@ skipping 76 matching lines @@
   INSTANCE_METHOD,
   LOCAL_FUNCTION,
   CLOSURE,
 }
 
 class FunctionDeclStructure<R, A> extends DeclStructure<R, A> {
   final FunctionKind kind;
 
   FunctionDeclStructure(this.kind, FunctionElement function) : super(function);
 
+  // ignore: MISSING_RETURN
   R dispatch(SemanticDeclarationVisitor<R, A> visitor, FunctionExpression node,
       A arg) {
     switch (kind) {
       case FunctionKind.TOP_LEVEL_GETTER:
         return visitor.visitTopLevelGetterDeclaration(
             node, element, node.body, arg);
       case FunctionKind.TOP_LEVEL_SETTER:
         return visitor.visitTopLevelSetterDeclaration(
             node, element, node.parameters, node.body, arg);
       case FunctionKind.TOP_LEVEL_FUNCTION:
@@ skipping 221 matching lines @@
       return internalError(node, "Unexpected variable $element.");
     }
     if (element.isConst) {
       ConstantExpression constant = elements.getConstant(element.initializer);
       return new ConstantVariableStructure(kind, node, element, constant);
     } else {
       return new NonConstantVariableStructure(kind, node, element);
     }
   }
 }