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

sdk/lib/_internal/compiler/implementation/tree_validator.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
@@ skipping 40 matching lines @@
         expect(node.arguments.tail.head, node.arguments.tail.isEmpty);
       } else {
         expect(node.arguments.head, node.arguments.isEmpty);
       }
     } else {
       expect(node, !node.arguments.isEmpty);
     }
   }
 
   visitReturn(Return node) {
-    if (!node.isRedirectingFactoryBody && node.hasExpression) {
+    if (node.hasExpression) {
       // We allow non-expression expressions in Return nodes, but only when
       // using them to hold redirecting factory constructors.
       expect(node, node.expression.asExpression() != null);
     }
   }
 }
 
 class InvalidNodeError {
   final Node node;
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = node.toDebugString();
     String result = 'invalid node: $nodeString';
     if (message != null) result = '$result ($message)';
     return result;
   }
 }