improve AST validation (#3864)

lib/ast.js

@@ -784,6 +784,7 @@ var AST_Sequence = DEFNODE("Sequence", "expressions", {
         });
     },
     _validate: function() {
+        if (this.expressions.length < 2) throw new Error("expressions must contain multiple elements");
         must_be_expressions(this, "expressions");
     },
 });