[es5] call ToString() on argument in String.prototype.concat() fast case

15.5.4.6 5.b requires each part to be converted using ToString(). This also needs to occur in the single argument fast-case.

BUG=v8:3981
R=arv@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1029103002

Cr-Commit-Position: refs/heads/master@{#27372}

src/string.js

@@ -72,11 +72,10 @@ function StringCharCodeAtJS(pos) {
 // ECMA-262, section 15.5.4.6
 function StringConcat(other /* and more */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
-
   var len = %_ArgumentsLength();
   var this_as_string = TO_STRING_INLINE(this);
   if (len === 1) {
-    return this_as_string + other;
+    return this_as_string + TO_STRING_INLINE(other);
   }
   var parts = new InternalArray(len + 1);
   parts[0] = this_as_string;

test/mjsunit/string-concat.js

+// Copyright 2015 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.
+
+function Stringified(toString) {
+  var valueOf = "-" + toString + "-";
+  return {
+    toString: function() { return toString; },
+    valueOf: function() { return valueOf; }
+  };
+}
+
+assertEquals("a.b.", "a.".concat(Stringified("b.")));
+assertEquals("a.b.c.", "a.".concat(Stringified("b."), Stringified("c.")));