Remove uses of string + from API document generator.

lib/dartdoc/block_parser.dart

Index: lib/dartdoc/block_parser.dart
diff --git a/lib/dartdoc/block_parser.dart b/lib/dartdoc/block_parser.dart
index f7e1f544527b092360b77487071befdfac30fe4c..09ce8a4bf24804acd2c7ef5e419022037134f3ea 100644
--- a/lib/dartdoc/block_parser.dart
+++ b/lib/dartdoc/block_parser.dart
@@ -20,8 +20,8 @@ final _RE_INDENT = const RegExp(@'^(?:    |\t)(.*)$');
 /// Three or more hyphens, asterisks or underscores by themselves. Note that
 /// a line like `----` is valid as both HR and SETEXT. In case of a tie,
 /// SETEXT should win.
-final _RE_HR = const RegExp(@'^[ ]{0,3}((-+[ ]{0,2}){3,}|' +
-                                 @'(_+[ ]{0,2}){3,}|' +
+final _RE_HR = const RegExp(@'^[ ]{0,3}((-+[ ]{0,2}){3,}|'
+                                 @'(_+[ ]{0,2}){3,}|'
                                  @'(\*+[ ]{0,2}){3,})$');
 
 /// Really hacky way to detect block-level embedded HTML. Just looks for