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_1_test.dart b/tests/language/propagate_past_constant_test.dart
similarity index 56%
copy from tests/language/regress_21998_1_test.dart
copy to tests/language/propagate_past_constant_test.dart
index 3237f3396629f6fd11270e1adc290bec78713958..dcb30eb74b2f3dfb4fa894030b0d59bd150e505b 100644
--- a/tests/language/regress_21998_1_test.dart
+++ b/tests/language/propagate_past_constant_test.dart
@@ -2,17 +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';
 
-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, 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);
+}