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

sdk/lib/_internal/compiler/implementation/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 89 matching lines @@
     ..asLiteralList()
     ..asLiteralMap()
     ..asLiteralMapEntry()
     ..asLiteralNull()
     ..asLiteralSymbol()
     ..asMetadata()
     ..asModifiers()
     ..asPart()
     ..asPartOf()
     ..asRethrow()
+    ..asReturn()
     ..asStatement()
     ..asStringInterpolation()
     ..asStringInterpolationPart()
     ..asStringJuxtaposition()
     ..asStringNode()
     ..asSwitchCase()
     ..asSwitchStatement()
     ..asTryStatement()
     ..asTypeAnnotation()
     ..asTypeVariable()
@@ skipping 105 matching lines @@
     ..visitParameter(null);
   task
     ..hasIr(null)
     ..getIr(null);
 }
 
 useCompiler(dart2jslib.Compiler compiler) {
   compiler.libraryLoader.reset();
   compiler.libraryLoader.lookupLibrary(null);
 }