Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/resolution/secret_tree_element.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Encapsulates the field [TreeElementMixin._element].
  *
  * This library is an implementation detail of dart2js, and should not
  * be imported except by resolution and tree node libraries, or for
  * testing.
  *
  * We have taken great care to ensure AST nodes can be cached between
  * compiler instances.  Part of this requires that we always access
  * resolution results through TreeElements.
  *
  * So please, do not add additional elements to this library, and do
  * not import it.
  */
 library secret_tree_element;
 
-import '../dart2jslib.dart' show invariant, Spannable;
+import '../dart2jslib.dart' show invariant;
+import '../util/util.dart' show Spannable;
 
 /// Interface for associating
 abstract class TreeElementMixin {
   Object get _element;
   void set _element(Object value);
 }
 
 /// Null implementation of [TreeElementMixin] which does not allow association
 /// of elements.
 ///
@@ skipping 26 matching lines @@
  */
 Object getTreeElement(TreeElementMixin node) => node._element;
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  */
 void setTreeElement(TreeElementMixin node, Object value) {
   node._element = value;
 }