Add branch to footer information in flutter docs (#20711)

dev/tools/dartdoc.dart

@@ -193,22 +193,31 @@ ArgParser _createArgsParser() {
   return parser;
 }
 
+final RegExp gitBranchRegexp = new RegExp(r'^## (.*)');
+
 void createFooter(String footerPath) {
   const int kGitRevisionLength = 10;
 
-  final ProcessResult gitResult = Process.runSync('git', <String>['rev-parse', 'HEAD']);
+  ProcessResult gitResult = Process.runSync('git', <String>['rev-parse', 'HEAD']);
   if (gitResult.exitCode != 0)
-    throw 'git exit with non-zero exit code: ${gitResult.exitCode}';
+    throw 'git rev-parse exit with non-zero exit code: ${gitResult.exitCode}';
   String gitRevision = gitResult.stdout.trim();
 
+  gitResult = Process.runSync('git', <String>['status', '-b', '--porcelain']);
+   if (gitResult.exitCode != 0)
+    throw 'git status exit with non-zero exit code: ${gitResult.exitCode}';
+  final Match gitBranchMatch = gitBranchRegexp.firstMatch(
+      gitResult.stdout.trim().split('\n').first);
+  final String gitBranchOut = gitBranchMatch == null ? '' : '• </span class="no-break">${gitBranchMatch.group(1)}</span>';
+
   gitRevision = gitRevision.length > kGitRevisionLength ? gitRevision.substring(0, kGitRevisionLength) : gitRevision;
 
   final String timestamp = new DateFormat('yyyy-MM-dd HH:mm').format(new DateTime.now());
 
-  new File(footerPath).writeAsStringSync(
-    '• </span class="no-break">$timestamp<span> '
-    '• </span class="no-break">$gitRevision</span>'
-  );
+  new File(footerPath).writeAsStringSync(<String>[
+    '• </span class="no-break">$timestamp<span>',
+    '• </span class="no-break">$gitRevision</span>',
+    gitBranchOut].join(' '));
 }
 
 void sanityCheckDocs() {