Use the built-in github extension set

...and notice pass rate goes down? Hrm…
diff --git a/pkgs/markdown/tool/gfm_stats.json b/pkgs/markdown/tool/gfm_stats.json
index 183aa1f..04e3836 100644
--- a/pkgs/markdown/tool/gfm_stats.json
+++ b/pkgs/markdown/tool/gfm_stats.json
@@ -1,22 +1,22 @@
 {
  "ATX headings": {
-  "32": "strict",
+  "32": "fail",
   "33": "strict",
   "34": "strict",
   "35": "strict",
-  "36": "strict",
-  "37": "strict",
+  "36": "fail",
+  "37": "fail",
   "38": "fail",
   "39": "strict",
   "40": "loose",
   "41": "fail",
-  "42": "strict",
-  "43": "loose",
-  "44": "strict",
-  "45": "loose",
-  "46": "loose",
-  "47": "strict",
-  "48": "strict",
+  "42": "fail",
+  "43": "fail",
+  "44": "fail",
+  "45": "fail",
+  "46": "fail",
+  "47": "fail",
+  "48": "fail",
   "49": "fail"
  },
  "Autolinks": {
@@ -69,14 +69,14 @@
   "308": "fail"
  },
  "Blank lines": {
-  "188": "strict"
+  "188": "fail"
  },
  "Block quotes": {
-  "196": "loose",
-  "197": "loose",
-  "198": "loose",
+  "196": "fail",
+  "197": "fail",
+  "198": "fail",
   "199": "strict",
-  "200": "loose",
+  "200": "fail",
   "201": "loose",
   "202": "loose",
   "203": "loose",
@@ -285,7 +285,7 @@
   "106": "fail",
   "107": "loose",
   "108": "strict",
-  "109": "strict",
+  "109": "fail",
   "110": "strict",
   "111": "strict",
   "112": "strict",
@@ -306,8 +306,8 @@
   "636": "strict",
   "637": "strict",
   "638": "loose",
-  "639": "strict",
-  "640": "strict"
+  "639": "fail",
+  "640": "fail"
  },
  "HTML blocks": {
   "115": "strict",
@@ -386,7 +386,7 @@
   "81": "strict",
   "82": "loose",
   "83": "strict",
-  "84": "strict",
+  "84": "fail",
   "85": "strict",
   "86": "loose",
   "87": "strict"
@@ -415,7 +415,7 @@
   "174": "loose",
   "175": "strict",
   "176": "strict",
-  "177": "loose",
+  "177": "fail",
   "178": "strict",
   "179": "loose"
  },
@@ -553,7 +553,7 @@
   "265": "loose",
   "266": "loose",
   "267": "loose",
-  "268": "loose"
+  "268": "fail"
  },
  "Lists": {
   "271": "loose",
@@ -618,29 +618,29 @@
   "624": "fail"
  },
  "Setext headings": {
-  "50": "strict",
-  "51": "strict",
-  "52": "strict",
-  "53": "loose",
+  "50": "fail",
+  "51": "fail",
+  "52": "fail",
+  "53": "fail",
   "54": "strict",
-  "55": "strict",
+  "55": "fail",
   "56": "loose",
   "57": "strict",
-  "58": "loose",
-  "59": "strict",
-  "60": "loose",
+  "58": "fail",
+  "59": "fail",
+  "60": "fail",
   "61": "loose",
   "62": "fail",
   "63": "loose",
-  "64": "strict",
-  "65": "strict",
+  "64": "fail",
+  "65": "fail",
   "66": "strict",
   "67": "strict",
   "68": "loose",
   "69": "strict",
   "70": "loose",
-  "71": "loose",
-  "72": "strict",
+  "71": "fail",
+  "72": "fail",
   "73": "strict",
   "74": "strict",
   "75": "strict"
@@ -655,12 +655,12 @@
   "467": "strict"
  },
  "Tables (extension)": {
-  "189": "fail",
+  "189": "loose",
   "190": "fail",
-  "191": "fail",
+  "191": "loose",
   "192": "fail",
   "193": "fail",
-  "194": "strict",
+  "194": "fail",
   "195": "fail"
  },
  "Tabs": {
@@ -673,7 +673,7 @@
   "7": "loose",
   "8": "strict",
   "9": "fail",
-  "10": "strict",
+  "10": "fail",
   "11": "fail"
  },
  "Textual content": {
@@ -698,7 +698,7 @@
   "26": "loose",
   "27": "loose",
   "28": "strict",
-  "29": "strict",
+  "29": "fail",
   "30": "fail",
   "31": "loose"
  }
diff --git a/pkgs/markdown/tool/gfm_stats.txt b/pkgs/markdown/tool/gfm_stats.txt
index e5f625f..d741636 100644
--- a/pkgs/markdown/tool/gfm_stats.txt
+++ b/pkgs/markdown/tool/gfm_stats.txt
@@ -1,31 +1,31 @@
-  15 of   18 –  83.3%  ATX headings
+   5 of   18 –  27.8%  ATX headings
   15 of   19 –  78.9%  Autolinks
    0 of   11 –   0.0%  Autolinks (extension)
    8 of   13 –  61.5%  Backslash escapes
-   1 of    1 – 100.0%  Blank lines
-  22 of   25 –  88.0%  Block quotes
+   0 of    1 –   0.0%  Blank lines
+  18 of   25 –  72.0%  Block quotes
   14 of   16 –  87.5%  Code spans
    0 of    1 –   0.0%  Disallowed Raw HTML (extension)
   79 of  128 –  61.7%  Emphasis and strong emphasis
   10 of   12 –  83.3%  Entity and numeric character references
-  24 of   27 –  88.9%  Fenced code blocks
-  15 of   15 – 100.0%  Hard line breaks
+  23 of   27 –  85.2%  Fenced code blocks
+  13 of   15 –  86.7%  Hard line breaks
   42 of   42 – 100.0%  HTML blocks
   21 of   22 –  95.5%  Images
-  11 of   12 –  91.7%  Indented code blocks
+  10 of   12 –  83.3%  Indented code blocks
    1 of    1 – 100.0%  Inlines
-  20 of   23 –  87.0%  Link reference definitions
+  19 of   23 –  82.6%  Link reference definitions
   56 of   84 –  66.7%  Links
-  44 of   48 –  91.7%  List items
+  43 of   48 –  89.6%  List items
   18 of   24 –  75.0%  Lists
    8 of    8 – 100.0%  Paragraphs
    1 of    1 – 100.0%  Precedence
   15 of   21 –  71.4%  Raw HTML
-  25 of   26 –  96.2%  Setext headings
+  13 of   26 –  50.0%  Setext headings
    2 of    2 – 100.0%  Soft line breaks
    1 of    3 –  33.3%  Strikethrough (extension)
-   1 of    7 –  14.3%  Tables (extension)
-   6 of   11 –  54.5%  Tabs
+   2 of    7 –  28.6%  Tables (extension)
+   5 of   11 –  45.5%  Tabs
    3 of    3 – 100.0%  Textual content
-  17 of   19 –  89.5%  Thematic breaks
- 495 of  643 –  77.0%  TOTAL
+  16 of   19 –  84.2%  Thematic breaks
+ 461 of  643 –  71.7%  TOTAL
diff --git a/pkgs/markdown/tool/stats.dart b/pkgs/markdown/tool/stats.dart
index 3d17613..e6a785a 100644
--- a/pkgs/markdown/tool/stats.dart
+++ b/pkgs/markdown/tool/stats.dart
@@ -8,7 +8,7 @@
 import 'package:collection/collection.dart';
 import 'package:html/dom.dart';
 import 'package:html/parser.dart' show parseFragment;
-import 'package:markdown/markdown.dart' show markdownToHtml;
+import 'package:markdown/markdown.dart' show markdownToHtml, ExtensionSet;
 import 'package:path/path.dart' as p;
 
 // Locate the "tool" directory. Use mirrors so that this works with the test
@@ -28,7 +28,8 @@
         negatable: false)
     ..addFlag('verbose',
         defaultsTo: false, help: 'verbose output', negatable: false)
-    ..addOption('flavor', allowed: ['common_mark', 'gfm'], defaultsTo: 'common_mark')
+    ..addOption('flavor',
+        allowed: ['common_mark', 'gfm'], defaultsTo: 'common_mark')
     ..addFlag('help', defaultsTo: false, negatable: false);
 
   ArgResults options;
@@ -61,6 +62,11 @@
 
   final testPrefix = options['flavor'];
 
+  ExtensionSet extensionSet;
+  if (testPrefix == 'gfm') {
+    extensionSet = ExtensionSet.gitHub;
+  }
+
   var sections = _loadCommonMarkSections(testPrefix);
 
   var scores = new SplayTreeMap<String, SplayTreeMap<int, CompareLevel>>(
@@ -74,7 +80,8 @@
       var nestedMap = scores.putIfAbsent(
           section, () => new SplayTreeMap<int, CompareLevel>());
 
-      nestedMap[e.example] = _compareResult(e, verbose);
+      nestedMap[e.example] =
+          _compareResult(e, verbose, extensionSet: extensionSet);
     }
   });
 
@@ -87,10 +94,11 @@
   }
 }
 
-CompareLevel _compareResult(CommonMarkTestCase expected, bool verboseFail) {
+CompareLevel _compareResult(CommonMarkTestCase expected, bool verboseFail,
+    {ExtensionSet extensionSet}) {
   String output;
   try {
-    output = markdownToHtml(expected.markdown);
+    output = markdownToHtml(expected.markdown, extensionSet: extensionSet);
   } catch (err, stackTrace) {
     if (verboseFail) {
       printVerboseFailure(