Split analyzer2dart code into multiple source files

pkg/analyzer2dart/lib/src/closed_world.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 analyzer2dart.closedWorld;
+
+import 'package:analyzer/analyzer.dart';
+import 'package:analyzer/src/generated/element.dart';
+
+/**
+ * Container for the elements and AST nodes which have been determined by
+ * tree shaking to be reachable by the program being compiled.
+ */
+class ClosedWorld {
+  // TODO(paulberry): is it a problem to hold on to all the AST's for the
+  // duration of tree shaking & CPS generation?
+  Map<Element, AstNode> elements = <Element, AstNode>{};
+  ClosedWorld();
+}