Eliminate some DebugStep calls in async code

runtime/vm/scopes.h

Index: runtime/vm/scopes.h
===================================================================
--- runtime/vm/scopes.h	(revision 44565)
+++ runtime/vm/scopes.h	(working copy)
@@ -79,6 +79,11 @@
     is_captured_parameter_ = value;
   }
 
+  // By convention, internal variables start with a colon.
+  bool IsInternal() const {
+    return name_.CharAt(0) == ':';
+  }
+
   bool IsConst() const {
     return const_value_ != NULL;
   }