Fix debugger stub patching on x64 architecture

runtime/vm/instructions_x64.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Classes that describe assembly patterns as used by inline caches.
 
 #ifndef VM_INSTRUCTIONS_X64_H_
 #define VM_INSTRUCTIONS_X64_H_
 
 #ifndef VM_INSTRUCTIONS_H_
 #error Do not include instructions_ia32.h directly; use instructions.h instead.
@@ skipping 21 matching lines @@
   virtual bool IsValid() const {
     return TestBytesWith(pattern(), pattern_length_in_bytes());
   }
 
   // 'pattern' returns the expected byte pattern in form of an integer array
   // with length of 'pattern_length_in_bytes'. A '-1' element means 'any byte'.
   virtual const int* pattern() const = 0;
   virtual int pattern_length_in_bytes() const = 0;
 
   static intptr_t IndexFromPPLoad(uword start);
+  static intptr_t OffsetFromPPIndex(intptr_t index);
 
  protected:
   uword start() const { return start_; }
 
  private:
   // Returns true if the 'num_bytes' bytes at 'start_' correspond to
   // array of integers 'data'. 'data' elements are either a byte or -1, which
   // represents any byte.
   bool TestBytesWith(const int* data, int num_bytes) const;
 
@@ skipping 67 matching lines @@
   static const int kLengthInBytes = 5;
   virtual const int* pattern() const;
 
   DISALLOW_COPY_AND_ASSIGN(ShortCallPattern);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INSTRUCTIONS_X64_H_