Fix two issues with ATX header syntax

pkgs/markdown/lib/src/block_parser.dart

diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index 65d78ba..5a92064 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -16,7 +16,7 @@
 ///
 /// Starts with 1-6 unescaped `#` characters which must not be followed by a
 /// non-space character. Line may end with any number of `#` characters,.
-final _headerPattern = new RegExp(r'^(#{1,6})[ \x09\x0b\x0c](.*?)#*$');
+final _headerPattern = new RegExp(r'^ {0,3}(#{1,6})[ \x09\x0b\x0c](.*?)#*$');
 
 /// The line starts with `>` with one optional space after.
 final _blockquotePattern = new RegExp(r'^[ ]{0,3}>[ ]?(.*)$');

pkgs/markdown/tool/common_mark_stats.json

diff --git a/pkgs/markdown/tool/common_mark_stats.json b/pkgs/markdown/tool/common_mark_stats.json
index bfa5c58..bc14b36 100644
--- a/pkgs/markdown/tool/common_mark_stats.json
+++ b/pkgs/markdown/tool/common_mark_stats.json
@@ -6,10 +6,10 @@
   "35": "strict",
   "36": "strict",
   "37": "strict",
-  "38": "fail",
+  "38": "strict",
   "39": "strict",
   "40": "loose",
-  "41": "fail",
+  "41": "strict",
   "42": "strict",
   "43": "loose",
   "44": "strict",

pkgs/markdown/tool/common_mark_stats.txt

diff --git a/pkgs/markdown/tool/common_mark_stats.txt b/pkgs/markdown/tool/common_mark_stats.txt
index b854444..1d005a5 100644
--- a/pkgs/markdown/tool/common_mark_stats.txt
+++ b/pkgs/markdown/tool/common_mark_stats.txt
@@ -1,4 +1,4 @@
-  15 of   18 –  83.3%  ATX headings
+  17 of   18 –  94.4%  ATX headings
   19 of   19 – 100.0%  Autolinks
    9 of   13 –  69.2%  Backslash escapes
    1 of    1 – 100.0%  Blank lines
@@ -24,4 +24,4 @@
    6 of   11 –  54.5%  Tabs
    3 of    3 – 100.0%  Textual content
   17 of   19 –  89.5%  Thematic breaks
- 504 of  624 –  80.8%  TOTAL
+ 506 of  624 –  81.1%  TOTAL