@@ -10,6 +10,7 @@ const {
   ObjectDefineProperty,
   ObjectGetPrototypeOf,
   ObjectPrototypeHasOwnProperty,
+  SafeSet,
   String,
   StringPrototypeRepeat,
   StringPrototypeSlice,
@@ -42,7 +43,10 @@ const kReadableOperator = {
 const kMaxShortStringLength = 12;
 const kMaxLongStringLength = 512;
 
-const kMethodsWithCustomMessageDiff = ['deepStrictEqual', 'strictEqual', 'partialDeepStrictEqual'];
+const kMethodsWithCustomMessageDiff = new SafeSet()
+  .add('deepStrictEqual')
+  .add('strictEqual')
+  .add('partialDeepStrictEqual');
 
 function copyError(source) {
   const target = ObjectAssign(
@@ -263,7 +267,7 @@ class AssertionError extends Error {
     if (isErrorStackTraceLimitWritable()) Error.stackTraceLimit = 0;
 
     if (message != null) {
-      if (kMethodsWithCustomMessageDiff.includes(operator)) {
+      if (kMethodsWithCustomMessageDiff.has(operator)) {
         super(createErrDiff(actual, expected, operator, message, diff));
       } else {
         super(String(message));
@@ -283,7 +287,7 @@ class AssertionError extends Error {
         expected = copyError(expected);
       }
 
-      if (kMethodsWithCustomMessageDiff.includes(operator)) {
+      if (kMethodsWithCustomMessageDiff.has(operator)) {
         super(createErrDiff(actual, expected, operator, message, diff));
       } else if (operator === 'notDeepStrictEqual' ||
         operator === 'notStrictEqual') {