Revert of [runtime] Remove useless IN builtin.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index 25825d78131935a43c94a234d6c400e0c29f85c9..f7c91d3cc20e4130c4c6b20c46fe1c94483de68c 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -44,6 +44,7 @@
 var SAR_STRONG;
 var SHR;
 var SHR_STRONG;
+var IN;
 var INSTANCE_OF;
 var CALL_NON_FUNCTION;
 var CALL_NON_FUNCTION_AS_CONSTRUCTOR;
@@ -482,6 +483,21 @@
    -----------------------------
 */
 
+// ECMA-262, section 11.8.7, page 54.
+IN = function IN(x) {
+  if (!IS_SPEC_OBJECT(x)) {
+    throw %MakeTypeError(kInvalidInOperatorUse, this, x);
+  }
+  if (%_IsNonNegativeSmi(this)) {
+    if (IS_ARRAY(x) && %_HasFastPackedElements(x)) {
+      return this < x.length;
+    }
+    return %HasElement(x, this);
+  }
+  return %HasProperty(x, %$toName(this));
+}
+
+
 // ECMA-262, section 11.8.6, page 54. To make the implementation more
 // efficient, the return value should be zero if the 'this' is an
 // instance of F, and non-zero if not. This makes it possible to avoid