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

sdk/lib/_internal/compiler/implementation/tree/unparser.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.
 
 part of tree;
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
@@ skipping 348 matching lines @@
   }
 
   visitNodeList(NodeList node) {
     unparseNodeList(node);
   }
 
   visitOperator(Operator node) {
     visitIdentifier(node);
   }
 
+  visitRedirectingFactoryBody(RedirectingFactoryBody node) {
+    space();
+    write(node.beginToken.value);
+    space();
+    visit(node.constructorReference);
+    write(node.endToken.value);
+  }
+
   visitRethrow(Rethrow node) {
     write('rethrow;');
   }
 
   visitReturn(Return node) {
-    if (node.isRedirectingFactoryBody) {
-      write(' ');
-    }
     write(node.beginToken.value);
     if (node.hasExpression && node.beginToken.stringValue != '=>') {
       write(' ');
     }
     if (node.beginToken.stringValue == '=>') space();
     visit(node.expression);
     if (node.endToken != null) write(node.endToken.value);
   }
 
   unparseSendReceiver(Send node, {bool spacesNeeded: false}) {
@@ skipping 402 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }