warn if inlining the same css file more than once

pkg/polymer/lib/src/build/import_inliner.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.
 
 /// Transfomer that inlines polymer-element definitions from html imports.
 library polymer.src.build.import_inliner;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 13 matching lines @@
 import 'messages.dart';
 
 // TODO(sigmund): move to web_components package (dartbug.com/18037).
 class _HtmlInliner extends PolymerTransformer {
   final TransformOptions options;
   final Transform transform;
   final BuildLogger logger;
   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>');
 
   /// The number of extracted inline Dart scripts. Used as a counter to give
   /// unique-ish filenames.
   int inlineScriptCounter = 0;
 
   _HtmlInliner(TransformOptions options, Transform transform)
       : options = options,
@@ skipping 84 matching lines @@
           tag.remove();
           return null;
         }
         return _inlineImport(id, tag);
 
       } else if (rel == 'stylesheet') {
         if (id == null) return null;
         if (!options.shouldInlineStylesheet(id)) return null;
 
         changed = true;
+        if (inlinedStylesheetIds.contains(id)
+            && !options.stylesheetInliningIsOverridden(id)) {
+          logger.warning(
+              CSS_FILE_INLINED_MULTIPLE_TIMES.create({'url': id.path}),
+              span: tag.sourceSpan);
+        }
+        inlinedStylesheetIds.add(id);
         return _inlineStylesheet(id, tag);
       }
     }).then((_) => changed);
   }
 
   /// To preserve the order of scripts with respect to inlined
   /// link rel=import, we move both of those into the body before we do any
   /// inlining. We do not start doing this until the first import is found
   /// however, as some scripts do need to be ran in the head to work
   /// properly (platform.js for instance).
@@ skipping 396 matching lines @@
 /// style tag except these ones.
 const IGNORED_LINKED_STYLE_ATTRS =
     const ['charset', 'href', 'href-lang', 'rel', 'rev'];
 
 /// Global RegExp objects.
 final _INVALID_LIB_CHARS_REGEX = new RegExp('[^a-z0-9_]');
 final _NUM_REGEX = new RegExp('[0-9]');
 final _BINDING_REGEX = new RegExp(r'(({{.*}})|(\[\[.*\]\]))');
 
 _getSpan(SourceFile file, AstNode node) => file.span(node.offset, node.end);