opt out the tool/generate.dart script (dart-lang/term_glyph#19)

pkgs/term_glyph/pubspec.yaml

diff --git a/pkgs/term_glyph/pubspec.yaml b/pkgs/term_glyph/pubspec.yaml
index f4a49e8..d989ae9 100644
--- a/pkgs/term_glyph/pubspec.yaml
+++ b/pkgs/term_glyph/pubspec.yaml
@@ -10,4 +10,5 @@
 dev_dependencies:
   csv: '>=3.0.0 <5.0.0'
   dart_style: '>=0.2.0 <2.0.0'
+  meta: ^1.3.0-nullsafety
   test: ^1.16.0-nullsafety

pkgs/term_glyph/tool/generate.dart

diff --git a/pkgs/term_glyph/tool/generate.dart b/pkgs/term_glyph/tool/generate.dart
index 0a1136e..90fc2e8 100644
--- a/pkgs/term_glyph/tool/generate.dart
+++ b/pkgs/term_glyph/tool/generate.dart
@@ -1,10 +1,14 @@
 // Copyright (c) 2017, 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.
+//
+// TODO: Remove once package:csv/csv.dart is migrated.
+// @dart=2.9
 
 import 'dart:io';
 
 import 'package:csv/csv.dart';
+import 'package:meta/meta.dart';
 
 void main() {
   var csv = CsvCodec(eol: '\n');
@@ -82,7 +86,7 @@
 ///
 /// If [ascii] is `true`, this writes the ASCII glyph set. Otherwise it writes
 /// the Unicode glyph set.
-void _writeGlyphSet(List<List> data, {required bool ascii}) {
+void _writeGlyphSet(List<List> data, {@required bool ascii}) {
   var file =
       File('lib/src/generated/${ascii ? "ascii" : "unicode"}_glyph_set.dart')
           .openSync(mode: FileMode.write);