[es6] Use SubString in String{Starts,Ends}With

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 1bbc8a69ce1259515bb950d2b259c5269056435a..1db65bb0bad25b677c7fa38144976c6a39f4ca18 100644
--- a/src/string.js
+++ b/src/string.js
@@ -997,7 +997,7 @@ function StringStartsWith(searchString /* position */) {  // length == 1
     return false;
   }
 
-  return %StringIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }
 
 
@@ -1028,7 +1028,7 @@ function StringEndsWith(searchString /* position */) {  // length == 1
     return false;
   }
 
-  return %StringLastIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }