Reduce use of getClosureInfoForMember and cleanup closure_test

tests/compiler/dart2js/equivalence/id_equivalence_helper.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/common_elements.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/entities.dart';
@@ skipping 107 matching lines @@
     Map<int, String> annotations = <int, String>{};
     actualMap.forEach((Id id, ActualData data) {
       annotations[data.sourceSpan.begin] = data.value;
     });
     return withAnnotations(annotations);
   }
 
   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} | ${data.value}';
       }
     });
     expectedMap.forEach((Id id, String expected) {
       if (!actualMap.containsKey(id)) {
         int offset = compiler.reporter
             .spanFromSpannable(
                 computeSpannable(elementEnvironment, mainUri, id))
             .begin;
         annotations[offset] = '${expected} | ---';
@@ skipping 108 matching lines @@
       id = new NodeId(annotation.offset);
       expected = text;
     } else {
       id = new ElementId(text.substring(0, colonPos));
       expected = text.substring(colonPos + 1);
     }
     map[id] = expected;
   }
   return map;
 }