support optional chaining operator (#4899)

lib/output.js

@@ -1473,6 +1473,7 @@ function OutputStream(options) {
         var self = this;
         print_annotation(self, output);
         self.expression.print(output);
+        if (self.optional) output.print("?.");
         print_call_args(self, output);
     });
     DEFPRINT(AST_New, function(output) {
@@ -1501,22 +1502,23 @@ function OutputStream(options) {
         expr.print(output);
         var prop = self.property;
         if (output.option("ie8") && RESERVED_WORDS[prop]) {
-            output.print("[");
+            output.print(self.optional ? "?.[" : "[");
             output.add_mapping(self.end);
             output.print_string(prop);
             output.print("]");
         } else {
             if (expr instanceof AST_Number && !/[ex.)]/i.test(output.last())) output.print(".");
-            output.print(".");
+            output.print(self.optional ? "?." : ".");
             // the name after dot would be mapped about here.
             output.add_mapping(self.end);
             output.print_name(prop);
         }
     });
     DEFPRINT(AST_Sub, function(output) {
-        this.expression.print(output);
-        output.print("[");
-        this.property.print(output);
+        var self = this;
+        self.expression.print(output);
+        output.print(self.optional ? "?.[" : "[");
+        self.property.print(output);
         output.print("]");
     });
     DEFPRINT(AST_Spread, function(output) {