Move class DeclarationResolver to its own file.

pkg/analyzer/lib/src/dart/element/utilities.dart

-// Copyright (c) 2014, 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 analyzer.src.dart.element.utilities;
-
-import 'dart:collection';
-
-import 'package:analyzer/dart/element/element.dart';
-import 'package:analyzer/dart/element/visitor.dart';
-
-/**
- * A visitor that can be used to collect all of the non-synthetic elements in an
- * element model.
- */
-class ElementGatherer extends GeneralizingElementVisitor {
-  /**
-   * The set in which the elements are collected.
-   */
-  final Set<Element> elements = new HashSet<Element>();
-
-  /**
-   * Initialize the visitor.
-   */
-  ElementGatherer();
-
-  @override
-  void visitElement(Element element) {
-    if (!element.isSynthetic) {
-      elements.add(element);
-    }
-    super.visitElement(element);
-  }
-}