Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/native_arguments.h

Index: runtime/vm/native_arguments.h
diff --git a/runtime/vm/native_arguments.h b/runtime/vm/native_arguments.h
index 7035a50b53cfc1c87b2203d711f328c16343390b..67b1f72773b775237316342674247658d0327b8f 100644
--- a/runtime/vm/native_arguments.h
+++ b/runtime/vm/native_arguments.h
@@ -107,6 +107,18 @@ class NativeArguments {
     return *arg_ptr;
   }
 
+  void SetArgAt(int index, const Object& value) const {
+    ASSERT((index >= 0) && (index < ArgCount()));
+#if defined(TARGET_ARCH_DBC)
+    // On DBC stack is growing upwards, in reverse direction from all other
+    // architectures.
+    RawObject** arg_ptr = &(argv_[index]);

[Florian Schneider, 2016/08/10 02:32:14]

Since this is not used on DBC, maybe rather make it UNREACHABLE()?

[rmacnak, 2016/08/11 00:17:49]

Remove altogether after removing monomorphic case while in the middle of
precompiling.

+#else
+    RawObject** arg_ptr = &(argv_[-index]);
+#endif
+    *arg_ptr = value.raw();
+  }
+
   bool IsNativeAutoSetupScope() const {
     return AutoSetupScopeBits::decode(argc_tag_);
   }