Rename NativeReceiver to NativeArg0 as this method is used not necessarily

runtime/vm/native_arguments.h

Index: runtime/vm/native_arguments.h
===================================================================
--- runtime/vm/native_arguments.h	(revision 28028)
+++ runtime/vm/native_arguments.h	(working copy)
@@ -97,9 +97,14 @@
     return ArgCount() - NumHiddenArgs(function_bits);
   }
 
-  RawObject* NativeReceiver() const {
-    ASSERT(ToInstanceFunction());
-    return NativeArg0();
+  RawObject* NativeArg0() const {
+    int function_bits = FunctionBits::decode(argc_tag_);
+    if (function_bits == (kClosureFunctionBit | kInstanceFunctionBit)) {
+      // Retrieve the receiver from the context.
+      const Context& context = Context::Handle(isolate_->top_context());
+      return context.At(0);
+    }
+    return ArgAt(NumHiddenArgs(function_bits));
   }
 
   RawObject* NativeArgAt(int index) const {
@@ -201,16 +206,6 @@
     return 0;
   }
 
-  RawObject* NativeArg0() const {
-    int function_bits = FunctionBits::decode(argc_tag_);
-    if (function_bits == (kClosureFunctionBit | kInstanceFunctionBit)) {
-      // Retrieve the receiver from the context.
-      const Context& context = Context::Handle(isolate_->top_context());
-      return context.At(0);
-    }
-    return ArgAt(NumHiddenArgs(function_bits));
-  }
-
   Isolate* isolate_;  // Current isolate pointer.
   int argc_tag_;  // Encodes argument count and invoked native call type.
   RawObject*(*argv_)[];  // Pointer to an array of arguments to runtime call.