Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/equivalence/check_helpers.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.
 
 /// General equivalence test functions.
 
 library dart2js.equivalence.helpers;
 
 import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/constants/values.dart';
@@ skipping 71 matching lines @@
   @override
   bool testLists(
       Object object1, Object object2, String property, List list1, List list2,
       [bool elementEquivalence(a, b) = equality]) {
     return checkListEquivalence(object1, object2, property, list1, list2,
         (o1, o2, p, v1, v2) {
       if (!elementEquivalence(v1, v2)) {
         throw "$o1.$p = '${v1}' <> "
             "$o2.$p = '${v2}'";
       }
-      return true;
     });
   }
 
   @override
   bool testSets(
       var object1, var object2, String property, Iterable set1, Iterable set2,
       [bool elementEquivalence(a, b) = equality]) {
     return checkSetEquivalence(
         object1, object2, property, set1, set2, elementEquivalence);
   }
@@ skipping 374 matching lines @@
         (mismatch.isNotEmpty && failOnMismatch) ||
         remaining.isNotEmpty) {
       Expect.fail(message);
     } else {
       print(message);
     }
   } else if (verbose) {
     print(message);
   }
 }