Intrinsify String.charCodeAt, String.length and String.hashCode. Improves frog performance by 2x.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 1097)
+++ runtime/vm/object.h	(working copy)
@@ -2495,6 +2495,8 @@
     return *CharAddr(index);
   }
 
+  static intptr_t data_offset() { return OFFSET_OF(RawOneByteString, data_); }
+
   static intptr_t InstanceSize() {
     ASSERT(sizeof(RawOneByteString) == OFFSET_OF(RawOneByteString, data_));
     return 0;