Rename String.concat to operator+.

sdk/lib/_internal/dartdoc/lib/dartdoc.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * To generate docs for a library, run this script with the path to an
  * entrypoint .dart file, like:
  *
  *     $ dart dartdoc.dart foo.dart
  *
@@ skipping 346 matching lines @@
     var footerItems = [];
     if (!omitGenerationTime) {
       footerItems.add("This page was generated at ${new DateTime.now()}");
     }
     if (footerText != null) {
       footerItems.add(footerText);
     }
     var content = '';
     for (int i = 0; i < footerItems.length; i++) {
       if (i > 0) {
-        content = content.concat('\n');
+        content += '\n';
       }
-      content = content.concat('<div>${footerItems[i]}</div>');
+      content += '<div>${footerItems[i]}</div>';
     }
     return content;
   }
 
   void documentEntryPoint(Path entrypoint, Path libPath, Path pkgPath) {
     final compilation = new Compilation(entrypoint, libPath, pkgPath,
         COMPILER_OPTIONS);
     _document(compilation);
   }
 
@@ skipping 56 matching lines @@
     write(json_serializer.serialize(packageManifest));
     endFile();
 
     // Write out top level json file with a relative path to the library json
     // files.
     startFile("apidoc.json");
     packageManifest.location = revision;
     write(json_serializer.serialize(packageManifest));
     endFile();
   }
-  
+
   MdnComment lookupMdnComment(Mirror mirror) => null;
 
   void startFile(String path) {
     _filePath = new Path(path);
     _file = new StringBuffer();
   }
 
   void endFile() {
     final outPath = outputDir.join(_filePath);
     final dir = new Directory.fromPath(outPath.directoryPath);
@@ skipping 1560 matching lines @@
    return '''
         <div class="mdn">
         $mdnComment
         <div class="mdn-note"><a href="$mdnUrl">from MDN</a></div>
         </div>
         ''';
   }
 
   String toString() => mdnComment;
 }