Fix order of conversions in String.prototype.substr.

test/mjsunit/substr.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 134 matching lines @@
   last = z;
   cache.push(z);
   offset += i;
 }
 for (var i = 63; i >= 0; i--) {
   var z = cache.pop();
   assertTrue(/\u2028123456789ABCDEF/.test(z));
   assertEquals(xl - offset, z.length);
   offset -= i;
 }
+
+
+// Order of conversions.
+{
+  let log = [];
+  let string = {[Symbol.toPrimitive]() { log.push("this"); return "abc" }};
+  let start = {[Symbol.toPrimitive]() { log.push("start"); return 0 }};
+  let length = {[Symbol.toPrimitive]() { log.push("length"); return 1 }};
+  assertEquals("a", String.prototype.substr.call(string, start, length));
+  assertEquals(["this", "start", "length"], log);
+}
+{
+  let log = [];
+  let string = {[Symbol.toPrimitive]() { log.push("this"); return "abc" }};
+  let start = {[Symbol.toPrimitive]() { log.push("start"); return 0 }};
+  let length = {[Symbol.toPrimitive]() { log.push("length"); return 0 }};
+  assertEquals("", String.prototype.substr.call(string, start, length));
+  assertEquals(["this", "start", "length"], log);
+}