Propagate pseudo-constants expressions separately in tree rewriter.

tests/compiler/dart2js/js_backend_cps_ir_basic_test.dart

 // Copyright (c) 2014, 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 for basic functionality.
 
 library basic_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry(r"""
 main() {
   var e = 1;
   var l = [1, 2, 3];
   var m = {'s': 1};
 
   print('(' ')');
   print('(${true})');
   print('(${1})');
   print('(${[1, 2, 3]})');
   print('(${{'s': 1}})');
   print('($e)');
   print('($l)');
   print('($m)');
 }""",r"""
 function() {
-  var e, l, m;
-  e = 1;
+  var l, m;
   l = [1, 2, 3];
   m = P.LinkedHashMap_LinkedHashMap$_literal(["s", 1]);
   P.print("()");
   P.print("(" + true + ")");
   P.print("(" + 1 + ")");
   P.print("(" + H.S([1, 2, 3]) + ")");
   P.print("(" + H.S(P.LinkedHashMap_LinkedHashMap$_literal(["s", 1])) + ")");
-  P.print("(" + H.S(e) + ")");
+  P.print("(" + 1 + ")");
   P.print("(" + H.S(l) + ")");
   P.print("(" + H.S(m) + ")");
   return null;
 }"""),
   const TestEntry("""
 foo(a, [b = "b"]) => b;
 bar(a, {b: "b", c: "c"}) => c;
 main() {
   foo(0);
   foo(1, 2);
@@ skipping 81 matching lines @@
 function() {
   P.print(P.DateTime$now().isBefore$1(P.DateTime$now()));
   return null;
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }