Fix ToStringVisitor to handle external functions (issue 23968)

pkg/analyzer/test/parse_compilation_unit_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.parse.compilation.unit;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:unittest/unittest.dart';
 
 void main() {
@@ skipping 19 matching lines @@
       return error is AnalyzerErrorGroup &&
           error
               .toString()
               .contains("Error in <unknown source>: Expected to find ';'");
     })));
   });
 
   test("allows you to specify whether or not to parse function bodies", () {
     var unit = parseCompilationUnit("void main() => print('Hello, world!');",
         parseFunctionBodies: false);
-    expect(unit.toString(), equals("void main() ;"));
+    expect(unit.toString(), equals("void main();"));
   });
 }