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

tests/compiler/dart2js/compiler_helper.dart

Index: tests/compiler/dart2js/compiler_helper.dart
===================================================================
--- tests/compiler/dart2js/compiler_helper.dart	(revision 11018)
+++ tests/compiler/dart2js/compiler_helper.dart	(working copy)
@@ -57,10 +57,10 @@
 
 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) {