warn if inlining the same css file more than once

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

Index: pkg/polymer/lib/src/build/common.dart
diff --git a/pkg/polymer/lib/src/build/common.dart b/pkg/polymer/lib/src/build/common.dart
index 8ef384c70b70341c65f3dd73383e98c659c250bb..4d809f58177fac86205e83f76af5b5e0b4897756 100644
--- a/pkg/polymer/lib/src/build/common.dart
+++ b/pkg/polymer/lib/src/build/common.dart
@@ -131,6 +131,13 @@ class TransformOptions {
     var globalDefault = inlineStylesheets['default'];
     return (globalDefault != null) ? globalDefault : true;
   }
+
+  // Whether a stylesheet with [id] has an overriden inlining setting.
+  bool stylesheetInliningIsOverridden(AssetId id) {
+    return inlineStylesheets != null &&
+        (inlineStylesheets.containsKey(id.toString())
+          || inlineStylesheets.containsKey(id.path));
+  }
 }
 
 /// Mixin for polymer transformers.