warn if inlining the same css file more than once

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Common methods used by transfomers.
 library polymer.src.build.common;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 113 matching lines @@
     // First check for the full asset path overrides.
     var override = inlineStylesheets[id.toString()];
     if (override != null) return override;
     // Then check just the path overrides (if the package was not specified).
     override = inlineStylesheets[id.path];
     if (override != null) return override;
     // Then check the global default setting.
     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.
 abstract class PolymerTransformer {
   TransformOptions get options;
 
   Future<Document> readPrimaryAsHtml(Transform transform, BuildLogger logger) {
     var asset = transform.primaryInput;
     var id = asset.id;
     return asset.readAsString().then((content) {
@@ skipping 83 matching lines @@
 /// Returns true if this is a valid custom element name. See:
 /// <http://w3c.github.io/webcomponents/spec/custom/#dfn-custom-element-type>
 bool isCustomTagName(String name) {
   if (name == null || !name.contains('-')) return false;
   return !invalidTagNames.containsKey(name);
 }
 
 /// Regex to split names in the 'attributes' attribute, which supports 'a b c',
 /// 'a,b,c', or even 'a b,c'. This is the same as in `lib/src/declaration.dart`.
 final ATTRIBUTES_REGEX = new RegExp(r'\s|,');