Reduce instruction size on X64.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
===================================================================
--- runtime/vm/assembler_x64.cc	(revision 45219)
+++ runtime/vm/assembler_x64.cc	(working copy)
@@ -2555,7 +2555,8 @@
 
 void Assembler::j(Condition condition, const ExternalLabel* label) {
   Label no_jump;
-  j(static_cast<Condition>(condition ^ 1), &no_jump);  // Negate condition.
+  // Negate condition.
+  j(static_cast<Condition>(condition ^ 1), &no_jump, Assembler::kNearJump);
   jmp(label);
   Bind(&no_jump);
 }
@@ -2564,7 +2565,8 @@
 void Assembler::J(Condition condition, const ExternalLabel* label,
                   Register pp) {
   Label no_jump;
-  j(static_cast<Condition>(condition ^ 1), &no_jump);  // Negate condition.
+  // Negate condition.
+  j(static_cast<Condition>(condition ^ 1), &no_jump, Assembler::kNearJump);
   Jmp(label, pp);
   Bind(&no_jump);
 }