Fix mac x64 debug build by using fixed-length encoding.

runtime/vm/flow_graph_compiler_x64.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/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/ast_printer.h"
@@ skipping 1270 matching lines @@
            expected);
   }
 #endif  // DEBUG
 }
 
 
 int32_t FlowGraphCompiler::EdgeCounterIncrementSizeInBytes() {
   // Used by CodePatcher; so must be constant across all code in an isolate.
   int32_t size = 5;
 #if defined(DEBUG)
-  size += 33;  // VerifySmi
+  size += 36;  // VerifySmi
 #endif  // DEBUG
   if (VerifiedMemory::enabled()) {
     size += 72;
   }
   return size;
 }
 
 
 void FlowGraphCompiler::EmitOptimizedInstanceCall(
     ExternalLabel* target_label,
@@ skipping 511 matching lines @@
   __ movups(reg, Address(RSP, 0));
   __ AddImmediate(RSP, Immediate(kFpuRegisterSize), PP);
 }
 
 
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64