When replacing an instruction with another in GVN-like optimizations, try to find a better user of …

tests/compiler/dart2js/compiler_helper.dart

 // Copyright (c) 2012, 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.
 // Test constant folding.
 
 #library("compiler_helper");
 
 #import("dart:uri");
 #import("../../../lib/compiler/implementation/leg.dart", prefix: "leg");
 #import("../../../lib/compiler/implementation/js_backend/js_backend.dart", prefix: "js");
@@ skipping 39 matching lines @@
 String getIntTypeCheck(String variable) {
   return "\\($variable !== \\($variable \\| 0\\)\\)";
 }
 
 String getNumberTypeCheck(String variable) {
   return "\\(typeof $variable !== 'number'\\)";
 }
 
 bool checkNumberOfMatches(Iterator it, int nb) {
   for (int i = 0; i < nb; i++) {
-    Expect.isTrue(it.hasNext());
+    Expect.isTrue(it.hasNext(), "Found less than $nb matches");
     it.next();
   }
-  Expect.isFalse(it.hasNext());
+  Expect.isFalse(it.hasNext(), "Found more than $nb matches");
 }
 
 void compileAndMatch(String code, String entry, RegExp regexp) {
   String generated = compile(code, entry);
   Expect.isTrue(regexp.hasMatch(generated),
                 '"$generated" does not match /$regexp/');
 }
 
 void compileAndDoNotMatch(String code, String entry, RegExp regexp) {
   String generated = compile(code, entry);
   Expect.isFalse(regexp.hasMatch(generated),
                  '"$generated" has a match in /$regexp/');
 }