Introduce a class encapsulating arguments descriptor arrays.

runtime/vm/flow_graph_inliner.cc

Index: runtime/vm/flow_graph_inliner.cc
diff --git a/runtime/vm/flow_graph_inliner.cc b/runtime/vm/flow_graph_inliner.cc
index 5b39c10d9ae4adba61a37ee3057d88a0224bd4d1..2b3bfe2d1a92e7d6f6e1de95d8f08ce0a7ce4236 100644
--- a/runtime/vm/flow_graph_inliner.cc
+++ b/runtime/vm/flow_graph_inliner.cc
@@ -430,11 +430,12 @@ class CallSiteInliner : public ValueObject {
                                     arguments,
                                     &param_stubs,
                                     callee_graph);
-        // Add a bogus parameter at the end for the (unused) argument descriptor
-        // slot. The parser allocates an extra slot between locals and
-        // parameters to hold the argument descriptor in case it escapes.  We
-        // currently bailout if there are argument test expressions or escaping
-        // variables so this parameter and the stack slot are not used.
+        // Add a bogus parameter at the end for the (unused) arguments
+        // descriptor slot. The parser allocates an extra slot between
+        // locals and parameters to hold the arguments descriptor in case it
+        // escapes.  We currently bailout if there are argument test
+        // expressions or escaping variables so this parameter and the stack
+        // slot are not used.
         if (parsed_function->GetSavedArgumentsDescriptorVar() != NULL) {
           param_stubs.Add(new ParameterInstr(
               function.NumParameters(), callee_graph->graph_entry()));