Use rep_movsb for copying one-byte strings in substring intrinsic.

runtime/vm/disassembler_ia32.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_IA32)
 #include "platform/utils.h"
 #include "vm/allocation.h"
@@ skipping 896 matching lines @@
         GetModRm(*data, &mod, &regop, &rm);
         Print("movq ");
         PrintXmmRegister(regop);
         Print(",");
         data += PrintRightOperand(data);
         break;
       }
       default:
         UNIMPLEMENTED();
     }
+  } else if (*(data+1) == 0xA4) {
+    Print("rep_movsb");
+    data += 2;
+  } else {
+    UNIMPLEMENTED();
   }
   return data;
 }
 
 
 // Returns number of bytes used, including *data.
 int X86Decoder::F7Instruction(uint8_t* data) {
   ASSERT(*data == 0xF7);
   uint8_t modrm = *(data+1);
   int mod, regop, rm;
@@ skipping 875 matching lines @@
                                   pc);
     pc += instruction_length;
   }
 
   return;
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32