warn if inlining the same css file more than once

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

Index: pkg/polymer/lib/src/build/import_inliner.dart
diff --git a/pkg/polymer/lib/src/build/import_inliner.dart b/pkg/polymer/lib/src/build/import_inliner.dart
index b6a973f3e4ea39775df00de740df11bf42eacb0e..53c6810cbc4884200b1590dcc7b6bfbec3f5e5f4 100644
--- a/pkg/polymer/lib/src/build/import_inliner.dart
+++ b/pkg/polymer/lib/src/build/import_inliner.dart
@@ -31,6 +31,7 @@ class _HtmlInliner extends PolymerTransformer {
   final AssetId docId;
   final seen = new Set<AssetId>();
   final scriptIds = <AssetId>[];
+  final inlinedStylesheetIds = new Set<AssetId>();
   final extractedFiles = new Set<AssetId>();
   bool experimentalBootstrap = false;
   final Element importsWrapper = new Element.html('<div hidden></div>');
@@ -135,6 +136,12 @@ class _HtmlInliner extends PolymerTransformer {
         if (!options.shouldInlineStylesheet(id)) return null;
 
         changed = true;
+        if (inlinedStylesheetIds.contains(id)
+            && !options.stylesheetInliningIsOverridden(id)) {
+          logger.warning(CSS_FILE_INLINED_MULTIPLE_TIMES,
+              span: tag.sourceSpan);
+        }
+        inlinedStylesheetIds.add(id);
         return _inlineStylesheet(id, tag);
       }
     }).then((_) => changed);