Mix in [TreeElementMixin] only on nodes that need it.

sdk/lib/_internal/compiler/implementation/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;
 
-/**
- * The superclass of all AST nodes.
- */
-abstract class TreeElementMixin {
+import '../dart2jslib.dart' show invariant, Spannable;
+
+/// The superclass of all AST nodes.
+///
+/// This is the empty implementation which does not allow association of
+/// elements.
+abstract class TreeElementMixin implements Spannable {

[floitsch, 2014/07/25 12:30:08]

Let's go for an interface and two implementations: Null, and Stored.

[Johnni Winther, 2014/08/04 06:59:09]

Done.

   // Deliberately using [Object] here to thwart code completion.
   // You're not really supposed to access this field anyways.
+  Object get _element => null;
+  set _element(_) {
+    assert(invariant(this, false,
+        message: "Elements cannot be associated with ${runtimeType}."));
+  }
+}
+
+/// Actual implementation of [TreeElementMixin] which stores the associated
+/// element in the private field [_element].
+///
+/// This class is mixed into the nodes classes that are actually associated with
+/// elements.
+abstract class TreeElementMixinImpl implements TreeElementMixin {
   Object _element;
 }
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  *
  * Using [Object] as return type to thwart code completion.
  */
-Object getTreeElement(TreeElementMixin node) {
-  return node._element;
-}
+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;
 }