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

src/ast.cc

Index: src/ast.cc
diff --git a/src/ast.cc b/src/ast.cc
index 93cb6c09b99358f6a118f58e83442ef01f58e832..9dd825a6f96b96065806e9b8f489dd96364776a5 100644
--- a/src/ast.cc
+++ b/src/ast.cc
@@ -59,6 +59,12 @@ bool Expression::IsUndefinedLiteral(Isolate* isolate) const {
 }
 
 
+bool Expression::IsValidReferenceExpressionOrThis() const {
+  return IsValidReferenceExpression() ||
+         (IsVariableProxy() && AsVariableProxy()->is_this());
+}
+
+
 VariableProxy::VariableProxy(Zone* zone, Variable* var, int start_position,
                              int end_position)
     : Expression(zone, start_position),