Add support for GFM's strikethrough syntax (dart-lang/markdown#199) Add support for GFM's strikethrough syntax

diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index 306b2b1..dc28e7a 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md

@@ -3,6 +3,8 @@
 * Overhaul support for emphasis (`*foo*` and `_foo_`) and strong emphasis
   (`**foo**` and `__foo__`), dramatically improving CommonMark compliance.
 * Improve support for tab characters, and horizontal rules.
+* Add support for GitHub Flavored Markdown's Strikethrough extension. See the
+  [GFM spec][strikethrough].
 * The above fixes raise compliance with the CommonMark specs to 90%, and
   compliance with the GFM specs to 87%.
 * Allow the binary script to take a `--extension-set` option.
@@ -14,6 +16,7 @@
   $ markdown
   ```
 
+[strikethrough]: https://github.github.com/gfm/#strikethrough-extension-
 [pub-global]: https://www.dartlang.org/tools/pub/cmd/pub-global#running-a-script-from-your-path
 
 ## 1.1.1

diff --git a/pkgs/markdown/README.md b/pkgs/markdown/README.md
index 5fbe97c..fd1e11e 100644
--- a/pkgs/markdown/README.md
+++ b/pkgs/markdown/README.md

@@ -69,7 +69,7 @@
   * `new InlineHtmlSyntax()`
   * `const FencedCodeBlockSyntax()`
 
-* `ExtensionSet.gitHubWeb` includes six extensions:
+* `ExtensionSet.gitHubWeb` includes seven extensions:
 
   * `new EmojiSyntax()`
   * `new InlineHtmlSyntax()`
@@ -78,6 +78,7 @@
   * `const SetextHeaderWithIdSyntax()`, which adds `id` attributes to
     Setext-style headers, for easy intra-document linking.
   * `const FencedCodeBlockSyntax()`
+  * `new StrikethroughSyntax()`
   * `const TableSyntax()`
 
 ### Custom syntax extensions

diff --git a/pkgs/markdown/lib/src/extension_set.dart b/pkgs/markdown/lib/src/extension_set.dart
index 984ca6e..bab57e5 100644
--- a/pkgs/markdown/lib/src/extension_set.dart
+++ b/pkgs/markdown/lib/src/extension_set.dart

@@ -38,6 +38,7 @@
     const TableSyntax()
   ], [
     new InlineHtmlSyntax(),
+    new StrikethroughSyntax(),
     new EmojiSyntax(),
   ]);
 
@@ -45,9 +46,13 @@
   /// flavored Markdown spec].
   ///
   /// [GitHub flavored Markdown]: https://github.github.com/gfm/
-  static final ExtensionSet gitHubFlavored = new ExtensionSet(
-      [const FencedCodeBlockSyntax(), const TableSyntax()],
-      [new InlineHtmlSyntax()]);
+  static final ExtensionSet gitHubFlavored = new ExtensionSet([
+    const FencedCodeBlockSyntax(),
+    const TableSyntax()
+  ], [
+    new InlineHtmlSyntax(),
+    new StrikethroughSyntax(),
+  ]);
 
   /// The deprecated name for the [gitHubFlavored] extension set.
   @deprecated

diff --git a/pkgs/markdown/lib/src/inline_parser.dart b/pkgs/markdown/lib/src/inline_parser.dart
index fb6e8df..060d696 100644
--- a/pkgs/markdown/lib/src/inline_parser.dart
+++ b/pkgs/markdown/lib/src/inline_parser.dart

@@ -430,6 +430,25 @@
   }
 }
 
+/// Matches strikethrough syntax according to the GFM spec.
+class StrikethroughSyntax extends TagSyntax {
+  StrikethroughSyntax() : super('~+', requiresDelimiterRun: true);
+
+  @override
+  bool onMatchEnd(InlineParser parser, Match match, TagState state) {
+    var runLength = match.group(0).length;
+    var matchStart = parser.pos;
+    var matchEnd = parser.pos + runLength - 1;
+    var delimiterRun = _DelimiterRun.tryParse(parser, matchStart, matchEnd);
+    if (!delimiterRun.isRightFlanking) {
+      return false;
+    }
+
+    parser.addNode(new Element('del', state.children));
+    return true;
+  }
+}
+
 /// Matches inline links like `[blah][id]` and `[blah](url)`.
 class LinkSyntax extends TagSyntax {
   final Resolver linkResolver;

diff --git a/pkgs/markdown/pubspec.yaml b/pkgs/markdown/pubspec.yaml
index 031b66a..25fa227 100644
--- a/pkgs/markdown/pubspec.yaml
+++ b/pkgs/markdown/pubspec.yaml

@@ -9,11 +9,11 @@
 
 environment:
   sdk: '>=1.12.0 <2.0.0'
+
 dependencies:
   args: '^1.0.0'
 
 dev_dependencies:
-  args: '^1.0.0'
   # Only for example/app.dart.
   browser: any
   collection: '^1.2.0'

diff --git a/pkgs/markdown/tool/gfm_stats.json b/pkgs/markdown/tool/gfm_stats.json
index 5e0b8bb..ffa7b1c 100644
--- a/pkgs/markdown/tool/gfm_stats.json
+++ b/pkgs/markdown/tool/gfm_stats.json

@@ -653,8 +653,8 @@
   "646": "loose"
  },
  "Strikethrough (extension)": {
-  "469": "fail",
-  "470": "fail",
+  "469": "strict",
+  "470": "strict",
   "471": "strict"
  },
  "Tables (extension)": {

diff --git a/pkgs/markdown/tool/gfm_stats.txt b/pkgs/markdown/tool/gfm_stats.txt
index 656284c..0c472ca 100644
--- a/pkgs/markdown/tool/gfm_stats.txt
+++ b/pkgs/markdown/tool/gfm_stats.txt

@@ -23,9 +23,9 @@
   15 of   21 –  71.4%  Raw HTML
   25 of   26 –  96.2%  Setext headings
    2 of    2 – 100.0%  Soft line breaks
-   1 of    3 –  33.3%  Strikethrough (extension)
+   3 of    3 – 100.0%  Strikethrough (extension)
    2 of    8 –  25.0%  Tables (extension)
   11 of   11 – 100.0%  Tabs
    3 of    3 – 100.0%  Textual content
   19 of   19 – 100.0%  Thematic breaks
- 564 of  647 –  87.2%  TOTAL
+ 566 of  647 –  87.5%  TOTAL