Fix short jump address calculation in x64 disassembler

runtime/vm/disassembler_x64.cc

Index: runtime/vm/disassembler_x64.cc
diff --git a/runtime/vm/disassembler_x64.cc b/runtime/vm/disassembler_x64.cc
index dba7e427616823abff399d795765b05d8c9c303e..9f43a387ba81ce8d31f8280e3867d6eeec7713f4 100644
--- a/runtime/vm/disassembler_x64.cc
+++ b/runtime/vm/disassembler_x64.cc
@@ -878,7 +878,7 @@ int DisassemblerX64::JumpConditional(uint8_t* 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);