Enhance precision of recorded instantiation info in ResolutionWorldBuilder

tests/compiler/dart2js/serialization/test_helper.dart

Index: tests/compiler/dart2js/serialization/test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/test_helper.dart b/tests/compiler/dart2js/serialization/test_helper.dart
index f081bbdc1e2b68808285ffbe9db6264468d1466e..e0b8aaab8d6323e314998b1eb89aed67a97208c2 100644
--- a/tests/compiler/dart2js/serialization/test_helper.dart
+++ b/tests/compiler/dart2js/serialization/test_helper.dart
@@ -208,18 +208,20 @@ Set computeSetDifference(
   // set.difference would work)
   Set remaining = set2.toSet();
   for (var element1 in set1) {
+    bool found = false;
     var correspondingElement;
     for (var element2 in remaining) {
       if (sameElement(element1, element2)) {
         if (checkElements != null) {
           checkElements(element1, element2);
         }
+        found = true;
         correspondingElement = element2;
         remaining.remove(element2);
         break;
       }
     }
-    if (correspondingElement != null) {
+    if (found) {
       common.add([element1, correspondingElement]);
     } else {
       unfound.add(element1);