[strong] Allow mutually recursive classes.

src/variables.h

Index: src/variables.h
diff --git a/src/variables.h b/src/variables.h
index 545c3bd1f5bfae92e9adbe9c013e4dd65f2331fa..de7f39045af934d8a7d13320f58cbc8cb88588a2 100644
--- a/src/variables.h
+++ b/src/variables.h
@@ -18,7 +18,7 @@ namespace internal {
 
 class Variable: public ZoneObject {
  public:
-  enum Kind { NORMAL, FUNCTION, THIS, NEW_TARGET, ARGUMENTS };
+  enum Kind { NORMAL, FUNCTION, CLASS, THIS, NEW_TARGET, ARGUMENTS };
 
   enum Location {
     // Before and during variable allocation, a variable whose location is
@@ -97,6 +97,7 @@ class Variable: public ZoneObject {
   }
 
   bool is_function() const { return kind_ == FUNCTION; }
+  bool is_class() const { return kind_ == CLASS; }
   bool is_this() const { return kind_ == THIS; }
   bool is_new_target() const { return kind_ == NEW_TARGET; }
   bool is_arguments() const { return kind_ == ARGUMENTS; }