Add support for WritableRegister policy in the register allocator.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 20153ad438ceca507c377ac0b4605406633e2cd4..a8ce3afbcf2780816af0884bd707035418ffebb2 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -1594,11 +1594,9 @@ void Assembler::StoreIntoObjectFilter(Register object,
 void Assembler::StoreIntoObject(Register object,
                                 const FieldAddress& dest,
                                 Register value) {
-  // TODO(kmillikin): pass temp registers to avoid pushing registers.
+  ASSERT(object != value);
   movq(dest, value);
-
   Label done;
-  pushq(value);
   StoreIntoObjectFilter(object, value, &done);
   // A store buffer update is required.
   if (value != RAX) pushq(RAX);
@@ -1606,7 +1604,6 @@ void Assembler::StoreIntoObject(Register object,
   call(&StubCode::UpdateStoreBufferLabel());
   if (value != RAX) popq(RAX);
   Bind(&done);
-  popq(value);
 }