Make `String.prototype.{starts,ends}With` throw when passing a regular expression

src/harmony-string.js

Index: src/harmony-string.js
diff --git a/src/harmony-string.js b/src/harmony-string.js
index 8e4b9a462646470a4522c894547d2b0d81da1b6e..0b553720816a8cd86ab9deba3212c843d10ca6a1 100644
--- a/src/harmony-string.js
+++ b/src/harmony-string.js
@@ -64,6 +64,12 @@ function StringStartsWith(searchString /* position */) {  // length == 1
   }
 
   var s = TO_STRING_INLINE(this);
+
+  if (IS_REGEXP(searchString)) {
+    throw MakeTypeError("first_argument_not_regexp",
+                        ["String.prototype.startsWith"]);
+  }
+
   var ss = TO_STRING_INLINE(searchString);
   var pos = 0;
   if (%_ArgumentsLength() > 1) {
@@ -90,6 +96,12 @@ function StringEndsWith(searchString /* position */) {  // length == 1
   }
 
   var s = TO_STRING_INLINE(this);
+
+  if (IS_REGEXP(searchString)) {
+    throw MakeTypeError("first_argument_not_regexp",
+                        ["String.prototype.endsWith"]);
+  }
+
   var ss = TO_STRING_INLINE(searchString);
   var s_len = s.length;
   var pos = s_len;