assert: remove internal errorCache property

The internal assert module exposed an errorCache property solely for
testing. It is no longer necessary. Remove it.

Author: Trott

lib/assert.js

@@ -27,12 +27,14 @@ const { codes: {
   ERR_INVALID_ARG_VALUE,
   ERR_INVALID_RETURN_VALUE
 } } = require('internal/errors');
-const { AssertionError, errorCache } = require('internal/assert');
+const { AssertionError } = require('internal/assert');
 const { openSync, closeSync, readSync } = require('fs');
 const { inspect, types: { isPromise, isRegExp } } = require('util');
 const { EOL } = require('internal/constants');
 const { NativeModule } = require('internal/bootstrap/loaders');
 
+const errorCache = new Map();
+
 let isDeepEqual;
 let isDeepStrictEqual;
 let parseExpressionAt;

lib/internal/assert.js

@@ -396,6 +396,5 @@ class AssertionError extends Error {
 }
 
 module.exports = {
-  AssertionError,
-  errorCache: new Map()
+  AssertionError
 };