Use warning instead of error in a couple places. In polymer we are hitting

pkg/code_transformers/lib/assets.dart

Index: pkg/code_transformers/lib/assets.dart
diff --git a/pkg/code_transformers/lib/assets.dart b/pkg/code_transformers/lib/assets.dart
index 8fabaf6afd187f31c729bc9704e739be2b853156..f6643c78fd2eb24aeb0bdc34fad9ddc8b39e89a1 100644
--- a/pkg/code_transformers/lib/assets.dart
+++ b/pkg/code_transformers/lib/assets.dart
@@ -71,7 +71,7 @@ AssetId uriToAssetId(AssetId source, String url, TransformLogger logger,
       fixedSegments.addAll(sourceSegments.map((_) => '..'));
       fixedSegments.addAll(segments.sublist(index));
       var fixedUrl = urlBuilder.joinAll(fixedSegments);
-      logger.error('Invalid url to reach to another package: $url. Path '
+      logger.warning('Invalid url to reach to another package: $url. Path '
           'reaching to other packages must first reach up all the '
           'way to the $prefix folder. For example, try changing the url above '
           'to: $fixedUrl', span: span);
@@ -90,7 +90,7 @@ AssetId _extractOtherPackageId(int index, List segments,
   if (prefix != 'packages' && prefix != 'assets') return null;
   var folder = prefix == 'packages' ? 'lib' : 'asset';
   if (segments.length < index + 3) {
-    logger.error("incomplete $prefix/ path. It should have at least 3 "
+    logger.warning("incomplete $prefix/ path. It should have at least 3 "
         "segments $prefix/name/path-from-name's-$folder-dir", span: span);
     return null;
   }