Propagate pseudo-constants expressions separately in tree rewriter.

tests/language/propagate_past_constant_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.
 
-import 'dart:math' as Math;
 import 'package:expect/expect.dart';
 
-main() {
-  Expect.equals(4, new C().m());
+foo(x) => x;
+
+check(y) {
+        Expect.equals('foo', y);
 }
 
-class C {
-  max(a) => a;
-
-  m() {
-    return max(Math.max(2, 4));
-  }
-}
+main() {
+        var x = foo('foo');
+        var y = foo(x);
+        x = 'constant';
+        check(y); // 'y' should not propagate here unless reference to 'x' is rewritten
+        foo(x);
+        foo(x);
+}