[regexp] Properly handle HeapNumbers in AdvanceStringIndex

test/mjsunit/regress/regress-6209.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function testAdvanceStringIndex(lastIndex, expectedLastIndex) {
+  let exec_count = 0;
+  let last_last_index = -1;
+
+  let fake_re = {
+    exec: () => { return (exec_count++ == 0) ? [""] : null },
+    get lastIndex() { return lastIndex; },
+    set lastIndex(value) { last_last_index = value },
+    get global() { return true; },
+    get flags() { return "g"; }
+  };
+
+  assertEquals([""], RegExp.prototype[Symbol.match].call(fake_re, "abc"));
+  assertEquals(expectedLastIndex, last_last_index);
+}
+
+testAdvanceStringIndex(new Number(42), 43);  // Value wrapper.
+testAdvanceStringIndex(%AllocateHeapNumber(), 1);  // HeapNumber.
+testAdvanceStringIndex(4294967295, 4294967296);  // HeapNumber.