Drop all deprecated APIs (dart-lang/markdown#475)

A number of the deprecated classes in block_syntaxes were not exposed,
so not including those in the changelog
diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index a8b61a3..7c6672f 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md
@@ -2,21 +2,22 @@
 
 * **Breaking change**: `close()` of `DelimiterSyntax` and `LinkSyntax`
   returns multiple nodes instead of single one.
-
-## 6.0.2-dev
-
-* Fix a crash in checkbox lists when mixing checkbox items with
-  non-checkbox items.
-* Add a new syntax `BlockHtmlSyntax` to parse HTML blocks.
+* **Breaking change**: Remove deprecated APIs, including `TagSyntax`,
+  `indicatorForCheckedCheckBox`, and `indicatorForUncheckedCheckBox`.
+* **Breaking change**: Removed `BlockHtmlSyntax`, `BlockTagBlockHtmlSyntax`,
+  `LongBlockHtmlSyntax`, and `OtherTagBlockHtmlSyntax`.
+* Add a new syntax `HtmlBlockSyntax` to parse HTML blocks.
 * Add a new syntax `DecodeHtmlSyntax` to decode HTML entity and numeric
   character references.
 * Add a new syntax `SoftLineBreakSyntax` to remove the single space before the
   line ending.
-* Deprecate `BlockTagBlockHtmlSyntax`, `LongBlockHtmlSyntax` and
-  `OtherTagBlockHtmlSyntax`. These syntaxes will be removed from the next major
-  version.
 * Add an option `caseSensitive` to `TextSyntax`.
 
+## 6.0.1
+
+* Fix a crash in checkbox lists when mixing checkbox items with
+  non-checkbox items.
+
 ## 6.0.0
 
 * Require Dart 2.17
diff --git a/pkgs/markdown/lib/markdown.dart b/pkgs/markdown/lib/markdown.dart
index 58d0d8e..62d7576 100644
--- a/pkgs/markdown/lib/markdown.dart
+++ b/pkgs/markdown/lib/markdown.dart
@@ -38,7 +38,6 @@
 
 export 'src/ast.dart';
 export 'src/block_parser.dart';
-export 'src/block_syntaxes/block_html_syntax.dart';
 export 'src/block_syntaxes/block_syntax.dart';
 export 'src/block_syntaxes/blockquote_syntax.dart';
 export 'src/block_syntaxes/code_block_syntax.dart';
@@ -68,6 +67,7 @@
 export 'src/inline_syntaxes/autolink_syntax.dart';
 export 'src/inline_syntaxes/code_syntax.dart';
 export 'src/inline_syntaxes/color_swatch_syntax.dart';
+export 'src/inline_syntaxes/decode_html_syntax.dart';
 export 'src/inline_syntaxes/delimiter_syntax.dart';
 export 'src/inline_syntaxes/email_autolink_syntax.dart';
 export 'src/inline_syntaxes/emoji_syntax.dart';
@@ -80,7 +80,6 @@
 export 'src/inline_syntaxes/link_syntax.dart';
 export 'src/inline_syntaxes/soft_line_break_syntax.dart';
 export 'src/inline_syntaxes/strikethrough_syntax.dart';
-export 'src/inline_syntaxes/tag_syntax.dart';
 export 'src/inline_syntaxes/text_syntax.dart';
 
 const version = packageVersion;
diff --git a/pkgs/markdown/lib/src/block_syntaxes/block_html_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/block_html_syntax.dart
deleted file mode 100644
index 122c940..0000000
--- a/pkgs/markdown/lib/src/block_syntaxes/block_html_syntax.dart
+++ /dev/null
@@ -1,19 +0,0 @@
-// Copyright (c) 2022, 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.
-
-import '../block_parser.dart';
-import 'block_syntax.dart';
-
-/// Parses inline HTML at the block level. This differs from other Markdown
-/// implementations in several ways:
-///
-/// 1.  This one is way way WAY simpler.
-/// 2.  Essentially no HTML parsing or validation is done. We're a Markdown
-///     parser, not an HTML parser!
-abstract class BlockHtmlSyntax extends BlockSyntax {
-  @override
-  bool canEndBlock(BlockParser parser) => true;
-
-  const BlockHtmlSyntax();
-}
diff --git a/pkgs/markdown/lib/src/block_syntaxes/block_tag_block_html_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/block_tag_block_html_syntax.dart
deleted file mode 100644
index 7e40ac9..0000000
--- a/pkgs/markdown/lib/src/block_syntaxes/block_tag_block_html_syntax.dart
+++ /dev/null
@@ -1,50 +0,0 @@
-// Copyright (c) 2022, 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.
-
-import '../ast.dart';
-import '../block_parser.dart';
-import '../patterns.dart';
-import 'block_html_syntax.dart';
-
-@Deprecated('Use HtmlBlockSyntax instead')
-class BlockTagBlockHtmlSyntax extends BlockHtmlSyntax {
-  static final _pattern = RegExp(
-      '^ {0,3}</?(?:address|article|aside|base|basefont|blockquote|body|'
-      'caption|center|col|colgroup|dd|details|dialog|dir|div|dl|dt|fieldset|'
-      'figcaption|figure|footer|form|frame|frameset|h1|head|header|hr|html|'
-      'iframe|legend|li|link|main|menu|menuitem|meta|nav|noframes|ol|optgroup|'
-      'option|p|param|section|source|summary|table|tbody|td|tfoot|th|thead|'
-      'title|tr|track|ul)'
-      r'(?:\s|>|/>|$)');
-
-  /// The [_pattern] regular expression above is very expensive, even on
-  /// paragraphs of Markdown with no HTML. This regular expression can be used
-  /// first as a basic check that the input might possibly be an HTML block
-  /// tag, which occur very rarely in typical Markdown.
-  static final _openBracketPattern = RegExp('^ {0,3}<');
-
-  @override
-  RegExp get pattern => _pattern;
-
-  const BlockTagBlockHtmlSyntax();
-
-  @override
-  bool canParse(BlockParser parser) {
-    if (!_openBracketPattern.hasMatch(parser.current)) return false;
-    return super.canParse(parser);
-  }
-
-  @override
-  Node parse(BlockParser parser) {
-    final childLines = <String>[];
-
-    // Eat until we hit a blank line.
-    while (!parser.isDone && !parser.matches(emptyPattern)) {
-      childLines.add(parser.current);
-      parser.advance();
-    }
-
-    return Text(childLines.join('\n').trimRight());
-  }
-}
diff --git a/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
index 7b6d0b1..4105ec0 100644
--- a/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
+++ b/pkgs/markdown/lib/src/block_syntaxes/list_syntax.dart
@@ -9,18 +9,6 @@
 import 'ordered_list_with_checkbox_syntax.dart';
 import 'unordered_list_with_checkbox_syntax.dart';
 
-/// As of Markdown 6.0.1 invisible indicators for checked/unchecked checkboxes are
-/// no longer used.  These constants are now empty strings to reflect that.
-@Deprecated(
-    'This string is no longer used internally.  It will be removed in a future version.')
-const indicatorForUncheckedCheckBox = '';
-
-/// As of Markdown 6.0.1 invisible indicators for checked/unchecked checkboxes are
-/// no longer used.  These constants are now empty strings to reflect that.
-@Deprecated(
-    'This string is no longer used internally.  It be will be removed in a future version.')
-const indicatorForCheckedCheckBox = '';
-
 class ListItem {
   const ListItem(
     this.lines, {
diff --git a/pkgs/markdown/lib/src/block_syntaxes/long_block_html_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/long_block_html_syntax.dart
deleted file mode 100644
index 89e294a..0000000
--- a/pkgs/markdown/lib/src/block_syntaxes/long_block_html_syntax.dart
+++ /dev/null
@@ -1,36 +0,0 @@
-// Copyright (c) 2022, 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.
-
-import '../ast.dart';
-import '../block_parser.dart';
-import 'block_html_syntax.dart';
-
-/// A BlockHtmlSyntax that has a specific `endPattern`.
-///
-/// In practice this means that the syntax dominates; it is allowed to eat
-/// many lines, including blank lines, before matching its `endPattern`.
-@Deprecated('Use HtmlBlockSyntax instead')
-class LongBlockHtmlSyntax extends BlockHtmlSyntax {
-  @override
-  final RegExp pattern;
-  final RegExp _endPattern;
-
-  LongBlockHtmlSyntax(String pattern, String endPattern)
-      : pattern = RegExp(pattern),
-        _endPattern = RegExp(endPattern);
-
-  @override
-  Node parse(BlockParser parser) {
-    final childLines = <String>[];
-    // Eat until we hit [endPattern].
-    while (!parser.isDone) {
-      childLines.add(parser.current);
-      if (parser.matches(_endPattern)) break;
-      parser.advance();
-    }
-
-    parser.advance();
-    return Text(childLines.join('\n').trimRight());
-  }
-}
diff --git a/pkgs/markdown/lib/src/block_syntaxes/other_tag_block_html_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/other_tag_block_html_syntax.dart
deleted file mode 100644
index bcdc5c7..0000000
--- a/pkgs/markdown/lib/src/block_syntaxes/other_tag_block_html_syntax.dart
+++ /dev/null
@@ -1,26 +0,0 @@
-// Copyright (c) 2022, 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.
-
-import '../block_parser.dart';
-import 'block_tag_block_html_syntax.dart';
-
-@Deprecated('Use HtmlBlockSyntax instead')
-class OtherTagBlockHtmlSyntax extends BlockTagBlockHtmlSyntax {
-  @override
-  bool canEndBlock(BlockParser parser) => false;
-
-  // Really hacky way to detect "other" HTML. This matches:
-  //
-  // * any opening spaces
-  // * open bracket and maybe a slash ("<" or "</")
-  // * some word characters
-  // * either:
-  //   * a close bracket, or
-  //   * whitespace followed by not-brackets followed by a close bracket
-  // * possible whitespace and the end of the line.
-  @override
-  RegExp get pattern => RegExp(r'^ {0,3}</?\w+(?:>|\s+[^>]*>)\s*$');
-
-  const OtherTagBlockHtmlSyntax();
-}
diff --git a/pkgs/markdown/lib/src/inline_syntaxes/tag_syntax.dart b/pkgs/markdown/lib/src/inline_syntaxes/tag_syntax.dart
deleted file mode 100644
index 56b51f2..0000000
--- a/pkgs/markdown/lib/src/inline_syntaxes/tag_syntax.dart
+++ /dev/null
@@ -1,10 +0,0 @@
-// Copyright (c) 2022, 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.
-
-import 'delimiter_syntax.dart';
-
-@Deprecated('Use DelimiterSyntax instead')
-class TagSyntax extends DelimiterSyntax {
-  TagSyntax(super.pattern, {super.requiresDelimiterRun});
-}