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';
-import 'regress_21998_lib1.dart' as lib1;
+
+foo(x) => x;
+
+check(y) {
+  Expect.equals('foo', y);
+}
 
 main() {
-  Expect.equals(4, new C().m());
+  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);
 }
-
-class C {
-  max(a) => a;
-
-  m() {
-    return max(Math.max(2, lib1.max('a', 'b', 'cd').length));
-  }
-}