Throw if setting length of a string in strict mode.

test/mjsunit/strict-mode.js

Index: test/mjsunit/strict-mode.js
diff --git a/test/mjsunit/strict-mode.js b/test/mjsunit/strict-mode.js
index 9137f8d03a9652f6c974436bff62dc478a08ec57..69be19c28a708a8ce5894402bffe249592355952 100644
--- a/test/mjsunit/strict-mode.js
+++ b/test/mjsunit/strict-mode.js
@@ -944,3 +944,16 @@ repeat(10, function() { testAssignToUndefined(false); });
 
   assertEquals(o[7], 17);
 })();
+
+
+(function TestAssignmentToStringLength() {
+  "use strict";
+
+  var str_val = "string";
+  var str_obj = new String(str_val);
+  var str_cat = str_val + str_val + str_obj;
+
+  assertThrows(function() { str_val.length = 1; }, TypeError);
+  assertThrows(function() { str_obj.length = 1; }, TypeError);
+  assertThrows(function() { str_cat.length = 1; }, TypeError);
+})();