Alphabetize json output.

tools/html_json_doc/lib/html_to_json.dart

Index: tools/html_json_doc/lib/html_to_json.dart
diff --git a/tools/html_json_doc/lib/html_to_json.dart b/tools/html_json_doc/lib/html_to_json.dart
index ddfb5eb2ef2a6fc7c7b6b10a63464e56ee489b5a..718cf94a6717a1f5f9111756e75c31af1ed167e7 100644
--- a/tools/html_json_doc/lib/html_to_json.dart
+++ b/tools/html_json_doc/lib/html_to_json.dart
@@ -299,8 +299,11 @@ String prettyPrintJson(Object json, [String indentation = '']) {
       '$recursiveOutput'
       '\n$indentation]';
   } else if (json is Map) {
+    var keys = json.keys
+      ..sort();
+
     // TODO(amouravski): No newline after :
-    var mapList = json.keys.map((key) =>
+    var mapList = keys.map((key) =>
         '$indentation${JSON.stringify(key)}:\n'
         '${prettyPrintJson(json[key], '$indentation  ')}');
     var recursiveOutput = Strings.join(mapList, ',\n');
@@ -311,4 +314,4 @@ String prettyPrintJson(Object json, [String indentation = '']) {
     output = '$indentation${JSON.stringify(json)}';
   }
   return output;
-}
+}