Rework leaf calls as just simple C calls with a signature and not the NativeArgumentDescriptor.

vm/stub_code_x64_test.cc

Index: vm/stub_code_x64_test.cc
===================================================================
--- vm/stub_code_x64_test.cc	(revision 8859)
+++ vm/stub_code_x64_test.cc	(working copy)
@@ -19,7 +19,7 @@
 namespace dart {
 
 DECLARE_RUNTIME_ENTRY(TestSmiSub);
-DECLARE_LEAF_RUNTIME_ENTRY(TestLeafSmiAdd);
+DECLARE_LEAF_RUNTIME_ENTRY(RawObject*, TestLeafSmiAdd, RawObject*, RawObject*);
 
 
 static Function* CreateFunction(const char* name) {
@@ -76,23 +76,17 @@
 static void GenerateCallToCallLeafRuntimeStub(Assembler* assembler,
                                               int value1,
                                               int value2) {
-  const int argc = 2;
   const Smi& smi1 = Smi::ZoneHandle(Smi::New(value1));
   const Smi& smi2 = Smi::ZoneHandle(Smi::New(value2));
-  const Object& result = Object::ZoneHandle();
-  const Context& context = Context::ZoneHandle(Context::New(0));
-  ASSERT(context.isolate() == Isolate::Current());
   __ enter(Immediate(0));
-  __ LoadObject(CTX, context);
-  __ PushObject(result);  // Push Null object for return value.
-  __ PushObject(smi1);  // Push argument 1 smi1.
-  __ PushObject(smi2);  // Push argument 2 smi2.
-  ASSERT(kTestLeafSmiAddRuntimeEntry.argument_count() == argc);
+  __ ReserveAlignedFrameSpace(2 * kWordSize);
+  __ LoadObject(RAX, smi1);
+  __ movq(Address(RSP, 0), EAX);  // Push argument 1 smi1.
+  __ LoadObject(RAX, smi2);
+  __ movq(Address(RSP, kWordSize), EAX);  // Push argument 2 smi2.
   __ CallRuntime(kTestLeafSmiAddRuntimeEntry);  // Call SmiAdd runtime func.
-  __ AddImmediate(RSP, Immediate(argc * kWordSize));
-  __ popq(RAX);  // Pop return value from return slot.
   __ leave();
-  __ ret();
+  __ ret();  // Return value is in RAX.
 }