Reduce use of getClosureInfoForMember and cleanup closure_test

tests/compiler/dart2js/equivalence/id_equivalence_helper.dart

Index: tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
diff --git a/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart b/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
index d89841238a3617963abffbb19c11feb2bac2d5b9..13fb0160cc83f17dbc991454de79675100c4523d 100644
--- a/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
+++ b/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
@@ -125,9 +125,8 @@ class IdData {
   String get diffCode {
     Map<int, String> annotations = <int, String>{};
     actualMap.forEach((Id id, ActualData data) {
-      String expected = expectedMap[id];
+      String expected = expectedMap[id] ?? '';
       if (data.value != expected) {
-        expected ??= '---';
         annotations[data.sourceSpan.begin] = '${expected} | ${data.value}';
       }
     });