[doc_checker] Added logic to follow 308 (Permanent Redirect).

Bug: BLD-410
Change-Id: Ia41e014fc369e60d9323d30e0df91dddda252a5a
diff --git a/tools/doc_checker/lib/link_verifier.dart b/tools/doc_checker/lib/link_verifier.dart
index 854ead0..45db2ae 100644
--- a/tools/doc_checker/lib/link_verifier.dart
+++ b/tools/doc_checker/lib/link_verifier.dart
@@ -55,6 +55,16 @@
           sleep(new Duration(milliseconds: delay));
           continue;
         }
+
+        // Http client doesn't automatically follow 308 (Permanent Redirect).
+        if (code == HttpStatus.permanentRedirect) {
+          if (response.headers.containsKey('location')) {
+            Uri redirectUri = Uri.parse(link.uri.origin + response.headers['location']);
+            return _verifyLink(new Link<P>(redirectUri, link.payload));
+          }
+          return false;
+        }
+
         return code == HttpStatus.ok;
       }
     } on IOException {