Represent implicit 'this' binding by 'super' in AST.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index b1a634edb4c7f02f4dc96658e3e9dba6c5cb9908..ee019e8f0fa60ee540fd9361029208bea07c27a8 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -339,6 +339,7 @@ class Expression : public AstNode {
     kTest
   };
 
+  // True iff the expression is a valid reference expression.
   virtual bool IsValidReferenceExpression() const { return false; }
 
   // Helpers for ToBoolean conversion.
@@ -362,6 +363,9 @@ class Expression : public AstNode {
   // True if we can prove that the expression is the undefined literal.
   bool IsUndefinedLiteral(Isolate* isolate) const;
 
+  // True iff the expression is a valid target for an assignment.
+  bool IsValidReferenceExpressionOrThis() const;
+
   // Expression type bounds
   Bounds bounds() const { return bounds_; }
   void set_bounds(Bounds bounds) { bounds_ = bounds; }