Revert "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 c9320a27e7c6bf084b7e4daa2190d7008dc1e481..c28be83cea3bf8904ef632d5ec2e78b9432f0daa 100644
--- a/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
+++ b/tests/compiler/dart2js/equivalence/id_equivalence_helper.dart
@@ -126,8 +126,9 @@ 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}';
       }
     });