Revert "Remove dependency on package:charcode (#66)" (#70)

This reverts commit 49ff31eabebed0da0ae6634124f8ba5c6fbf57f1.

Reverting to keep `lib/` directory content the same as the latest
published version.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae1c0d6..2c81256 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,3 @@
-# 1.8.0-nullsafety.5-dev
-
 # 1.8.0-nullsafety.4
 
 * Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release
diff --git a/lib/src/charcode.dart b/lib/src/charcode.dart
deleted file mode 100644
index 5182638..0000000
--- a/lib/src/charcode.dart
+++ /dev/null
@@ -1,15 +0,0 @@
-// Copyright (c) 2020, 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.
-
-/// "Carriage return" control character.
-const int $cr = 0x0D;
-
-/// "Line feed" control character.
-const int $lf = 0x0A;
-
-/// Space character.
-const int $space = 0x20;
-
-/// "Horizontal Tab" control character, common name.
-const int $tab = 0x09;
diff --git a/lib/src/highlighter.dart b/lib/src/highlighter.dart
index 5d28fc3..f622c4e 100644
--- a/lib/src/highlighter.dart
+++ b/lib/src/highlighter.dart
@@ -4,11 +4,11 @@
 
 import 'dart:math' as math;
 
+import 'package:charcode/charcode.dart';
 import 'package:collection/collection.dart';
 import 'package:path/path.dart' as p;
 import 'package:term_glyph/term_glyph.dart' as glyph;
 
-import 'charcode.dart';
 import 'colors.dart' as colors;
 import 'location.dart';
 import 'span.dart';
diff --git a/lib/src/span.dart b/lib/src/span.dart
index ba8bec7..94c05ba 100644
--- a/lib/src/span.dart
+++ b/lib/src/span.dart
@@ -2,10 +2,10 @@
 // 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.
 
+import 'package:charcode/charcode.dart';
 import 'package:path/path.dart' as p;
 import 'package:term_glyph/term_glyph.dart' as glyph;
 
-import 'charcode.dart';
 import 'file.dart';
 import 'highlighter.dart';
 import 'location.dart';
diff --git a/pubspec.yaml b/pubspec.yaml
index 9f7fca3..a5db06e 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_span
-version: 1.8.0-nullsafety.5-dev
+version: 1.8.0-nullsafety.4
 
 description: A library for identifying source spans and locations.
 homepage: https://github.com/dart-lang/source_span
@@ -8,6 +8,7 @@
   sdk: ">=2.12.0-0.0 <3.0.0"
 
 dependencies:
+  charcode: '>=1.2.0-nullsafety <1.2.0'
   collection: '>=1.15.0-nullsafety <1.15.0'
   path: '>=1.8.0-nullsafety <1.8.0'
   term_glyph: '>=1.2.0-nullsafety <1.2.0'