Propagate pseudo-constants expressions separately in tree rewriter.

tests/language/propagate_past_constant_test.dart

Index: tests/language/propagate_past_constant_test.dart
diff --git a/tests/language/regress_21998_2_test.dart b/tests/language/propagate_past_constant_test.dart
similarity index 55%
copy from tests/language/regress_21998_2_test.dart
copy to tests/language/propagate_past_constant_test.dart
index 53cace0ad782e5702a3f63c10369624363dd1984..03858b4b56aa238de47ad6417cc6e58a54e1cac3 100644
--- a/tests/language/regress_21998_2_test.dart
+++ b/tests/language/propagate_past_constant_test.dart
@@ -2,18 +2,19 @@
 // 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;
 
-main() {
-  Expect.equals(4, new C().m());
-}
+foo(x) => x;
 
-class C {
-  max(a) => a;
+check(y) {
+  Expect.equals('foo', y);
+}
 
-  m() {
-    return max(Math.max(2, lib1.max('a', 'b', 'cd').length));
-  }
-}
+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);
+}