Fix compiler error that wasn't showing up on my platform.

runtime/vm/deopt_instructions.cc

 // Copyright (c) 2013, 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/deopt_instructions.h"
 
 #include "vm/assembler.h"
 #include "vm/code_patcher.h"
 #include "vm/intermediate_language.h"
 #include "vm/locations.h"
@@ skipping 209 matching lines @@
       return false;
 
     case DeoptInstr::kRetAddress:
     case DeoptInstr::kPcMarker:
     case DeoptInstr::kCallerFp:
     case DeoptInstr::kCallerPc:
       return false;
 
     case DeoptInstr::kSuffix:
     case DeoptInstr::kMaterializeObject:
+    default:
       // We should not encounter these instructions when filling stack slots.
-      UNIMPLEMENTED();
+      UNREACHABLE();
       return false;
   }
+  UNREACHABLE();
+  return false;
 }
 
 
 void DeoptContext::FillDestFrame() {
   const Code& code = Code::Handle(code_);
   const DeoptInfo& deopt_info = DeoptInfo::Handle(deopt_info_);
 
   const intptr_t len = deopt_info.TranslationLength();
   GrowableArray<DeoptInstr*> deopt_instructions(len);
   const Array& deopt_table = Array::Handle(code.deopt_info_array());
@@ skipping 1159 matching lines @@
                           Smi* offset,
                           DeoptInfo* info,
                           Smi* reason) {
   intptr_t i = index * kEntrySize;
   *offset ^= table.At(i);
   *info ^= table.At(i + 1);
   *reason ^= table.At(i + 2);
 }
 
 }  // namespace dart