Propagate pseudo-constants expressions separately in tree rewriter.

tests/compiler/dart2js/js_backend_cps_ir_constructor_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Tests of interceptors.
 
 library constructor_test;
 
 import 'js_backend_cps_ir.dart';
@@ skipping 141 matching lines @@
 }
 class Foo extends Bar {
   Foo() : super('x', y: 'y', w: 'w');
 }
 main() {
   new Foo();
 }
 """,
 r"""
 function() {
-  var x, y, w, z, q, v0;
-  x = "x";
-  y = "y";
-  w = "w";
-  z = "z";
-  q = null;
+  var v0;
   v0 = new V.Foo();
-  v0.Bar$5$q$w$y$z(x, q, w, y, z);
+  v0.Bar$5$q$w$y$z("x", null, "w", "y", "z");
   return v0;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }