add tests for tc39/proposal-iterator-helpers#281 (#3869)

Co-authored-by: Ms2ger <Ms2ger@igalia.com>

test/built-ins/Iterator/from/iterable-primitives.js

@@ -37,3 +37,17 @@ assert.throws(TypeError, function () {
 assert.compareArray(Array.from(Iterator.from(new Number(5))), [0, 1, 2, 3, 4]);
 
 assert.compareArray(Array.from(Iterator.from('string')), ['s', 't', 'r', 'i', 'n', 'g']);
+
+const originalStringIterator = String.prototype[Symbol.iterator];
+let observedType;
+Object.defineProperty(String.prototype, Symbol.iterator, {
+  get() {
+    'use strict';
+    observedType = typeof this;
+    return originalStringIterator;
+  }
+});
+Iterator.from('');
+assert.sameValue(observedType, 'string');
+Iterator.from(new String(''));
+assert.sameValue(observedType, 'object');

test/built-ins/Iterator/from/supports-iterable.js

@@ -12,3 +12,4 @@ features: [iterator-helpers]
 flags: []
 ---*/
 assert.compareArray(Array.from(Iterator.from([0, 1, 2, 3])), [0, 1, 2, 3]);
+assert.compareArray(Array.from(Iterator.from(new String('str'))), ['s', 't', 'r']);