html escape log messages

pkg/polymer/lib/src/build/wrapped_logger.dart

Index: pkg/polymer/lib/src/build/wrapped_logger.dart
diff --git a/pkg/polymer/lib/src/build/wrapped_logger.dart b/pkg/polymer/lib/src/build/wrapped_logger.dart
index c732b75b73c3978be79fc12817242c46e1c59bc4..bd6d4f6dea94f87ae7919360cf4aa67c86c6ac23 100644
--- a/pkg/polymer/lib/src/build/wrapped_logger.dart
+++ b/pkg/polymer/lib/src/build/wrapped_logger.dart
@@ -87,7 +87,7 @@ class WrappedLogger implements TransformLogger {
                SourceSpan span) {
     var data = {
         'level': level.name,
-        'message': message,
+        'message': const HtmlEscape().convert(message),
     };
     if (assetId != null) {
       data['assetId'] = {
@@ -103,4 +103,4 @@ class WrappedLogger implements TransformLogger {
     }
     _logs.add(data);
   }
-}
+}