Add support for leaf runtime calls

vm/runtime_entry_test.cc

Index: vm/runtime_entry_test.cc
===================================================================
--- vm/runtime_entry_test.cc	(revision 8520)
+++ vm/runtime_entry_test.cc	(working copy)
@@ -41,4 +41,17 @@
   arguments.SetReturn(Smi::Handle(Smi::New(result)));
 }
 
+
+// A leaf runtime call for test purposes.
+// Arg0: a smi.
+// Arg1: a smi.
+// returns a smi representing arg0 + arg1.
+DEFINE_LEAF_RUNTIME_ENTRY(TestLeafSmiAdd, 2) {
+  ASSERT(args.Count() == kTestLeafSmiAddRuntimeEntry.argument_count());
+  // Ignoring overflow in the calculation below.
+  intptr_t result = Smi::Value(reinterpret_cast<RawSmi*>(args.At(0))) +
+      Smi::Value(reinterpret_cast<RawSmi*>(args.At(1)));
+  return Smi::New(result);
+}
+
 }  // namespace dart