Revert of [intrinsic] Drop the %_ValueOf intrinsic.

src/js/string.js

Index: src/js/string.js
diff --git a/src/js/string.js b/src/js/string.js
index 5180e3056fa55c374531eaf0545a99b109f0f321..b3bc3933cda29b04a3253f74d6949c85419f7231 100644
--- a/src/js/string.js
+++ b/src/js/string.js
@@ -36,6 +36,24 @@
 });
 
 //-------------------------------------------------------------------
+
+// ECMA-262 section 15.5.4.2
+function StringToString() {
+  if (!IS_STRING(this) && !IS_STRING_WRAPPER(this)) {
+    throw MakeTypeError(kNotGeneric, 'String.prototype.toString');
+  }
+  return %_ValueOf(this);
+}
+
+
+// ECMA-262 section 15.5.4.3
+function StringValueOf() {
+  if (!IS_STRING(this) && !IS_STRING_WRAPPER(this)) {
+    throw MakeTypeError(kNotGeneric, 'String.prototype.valueOf');
+  }
+  return %_ValueOf(this);
+}
+
 
 // ECMA-262, section 15.5.4.6
 function StringConcat(other /* and more */) {  // length == 1
@@ -738,6 +756,8 @@
 
 // Set up the non-enumerable functions on the String prototype object.
 utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
+  "valueOf", StringValueOf,
+  "toString", StringToString,
   "codePointAt", StringCodePointAt,
   "concat", StringConcat,
   "endsWith", StringEndsWith,