Fix ToStringVisitor to handle external functions (issue 23968)

pkg/analyzer/test/parse_compilation_unit_test.dart

Index: pkg/analyzer/test/parse_compilation_unit_test.dart
diff --git a/pkg/analyzer/test/parse_compilation_unit_test.dart b/pkg/analyzer/test/parse_compilation_unit_test.dart
index 0e2d3de82d844dbdb2fb7317b0e26ce63fb36c3b..fca4bd277d484e290e8b9eb3359716afe22ad155 100644
--- a/pkg/analyzer/test/parse_compilation_unit_test.dart
+++ b/pkg/analyzer/test/parse_compilation_unit_test.dart
@@ -37,6 +37,6 @@ void main() {
   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();"));
   });
 }