Fix beginning of line detection in `AutolinkExtensionSyntax` (dart-lang/markdown#555)

pkgs/markdown/CHANGELOG.md

diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index 6f275c9..3cd2f7a 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md
@@ -1,6 +1,7 @@
 ## 7.1.2-wip
 
 * Update all CommonMark specification links to 0.30.
+* Fix beginning of line detection in `AutolinkExtensionSyntax`. 
 
 ## 7.1.1
 

pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart

diff --git a/pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart b/pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart
index 85773dc..0333abb 100644
--- a/pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart
+++ b/pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart
@@ -60,12 +60,12 @@
       return false;
     }
 
-    // When it is a link and it is not preceded by `*`, `_`, `~`, `(`, or `>`,
-    // it is invalid. See
-    // https://github.github.com/gfm/#extended-autolink-path-validation.
+    // When it is a link and it is not at the beginning of a line, or preceded
+    // by whitespace, `*`, `_`, `~`, `(`, or `>`, it is invalid. See
+    // https://github.github.com/gfm/#autolinks-extension-.
     if (startMatch[1] != null && parser.pos > 0) {
       final precededBy = String.fromCharCode(parser.charAt(parser.pos - 1));
-      const validPrecedingChars = {' ', '*', '_', '~', '(', '>'};
+      const validPrecedingChars = {'\n', ' ', '*', '_', '~', '(', '>'};
       if (!validPrecedingChars.contains(precededBy)) {
         return false;
       }

pkgs/markdown/test/extensions/autolink_extension.unit

diff --git a/pkgs/markdown/test/extensions/autolink_extension.unit b/pkgs/markdown/test/extensions/autolink_extension.unit
index 8ab4613..4a07fdc 100644
--- a/pkgs/markdown/test/extensions/autolink_extension.unit
+++ b/pkgs/markdown/test/extensions/autolink_extension.unit
@@ -1,4 +1,10 @@
 >>> not a link 
 mhttp://www.foo.com
 <<<
-<p>mhttp://www.foo.com</p>
\ No newline at end of file
+<p>mhttp://www.foo.com</p>
+>>> following a newline
+m
+http://www.foo.com
+<<<
+<p>m
+<a href="http://www.foo.com">http://www.foo.com</a></p>
\ No newline at end of file