Fix short jump address calculation in x64 disassembler

runtime/vm/disassembler_x64.cc

 // 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.
 
 #include "vm/disassembler.h"
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_IA32.
 #if defined(TARGET_ARCH_X64)
 #include "platform/utils.h"
 #include "vm/allocation.h"
@@ skipping 860 matching lines @@
   AppendToBuffer("j%s ", mnem);
   AppendAddressToBuffer(dest);
   return 6;  // includes 0x0F
 }
 
 
 // Returns number of bytes used, including *data.
 int DisassemblerX64::JumpConditionalShort(uint8_t* data) {
   uint8_t cond = *data & 0x0F;
   uint8_t b = *(data + 1);
-  uint8_t* dest = data + static_cast<uint8_t>(b) + 2;
+  uint8_t* dest = data + static_cast<int8_t>(b) + 2;
   const char* mnem = conditional_code_suffix[cond];
   AppendToBuffer("j%s ", mnem);
   AppendAddressToBuffer(dest);
   return 2;
 }
 
 
 // Returns number of bytes used, including *data.
 int DisassemblerX64::SetCC(uint8_t* data) {
   ASSERT(0x0F == *data);
@@ skipping 1060 matching lines @@
   }
   hex_buffer[hex_index] = '\0';
   if (out_instr_len) {
     *out_instr_len = instruction_length;
   }
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64