Really really fix mac x64 debug build by using more fixed-length encoding and long jumps.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
===================================================================
--- runtime/vm/assembler_x64.cc	(revision 42740)
+++ runtime/vm/assembler_x64.cc	(working copy)
@@ -3059,7 +3059,8 @@
   COMPILE_ASSERT(kHeapObjectTag == 1);
   testb(dest, Immediate((kWordSize - 1) - kHeapObjectTag));
   j(ZERO, &ok, Assembler::kNearJump);
-  Stop("Expected heap object or Smi");
+  static const bool kFixedLengthEncoding = true;

[Florian Schneider, 2015/01/12 10:31:51]

It does not matter here, but I think we tend not to use static linkage for local
variables. There isn't any benefit here either, so I'd just drop static for this
local.

+  Stop("Expected heap object or Smi", kFixedLengthEncoding);
   Bind(&ok);
 }
 
@@ -3077,7 +3078,8 @@
 #endif
   cmpq(dest, Immediate(reinterpret_cast<uint64_t>(Object::null())));
   j(EQUAL, &ok, Assembler::kNearJump);
-  Stop("Expected zapped, Smi or null");
+  static const bool kFixedLengthEncoding = true;

[Florian Schneider, 2015/01/12 10:31:51]

Same.

+  Stop("Expected zapped, Smi or null", kFixedLengthEncoding);
   Bind(&ok);
 }