Only run [declareOutputs] once for each asset/transformer pair.

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 226 matching lines @@
 ///
 /// This transformer assumes that all script tags point to external files. To
 /// support script tags with inlined code, use this transformer after running
 /// [InlineCodeExtractor] on an earlier phase.
 class ImportInliner extends Transformer {
   final TransformOptions options;
 
   ImportInliner(this.options);
 
   /// Only run on entry point .html files.
-  Future<bool> isPrimary(Asset input) =>
-      new Future.value(options.isHtmlEntryPoint(input.id));
+  Future<bool> isPrimary(AssetId id) =>
+      new Future.value(options.isHtmlEntryPoint(id));
 
   Future apply(Transform transform) =>
       new _HtmlInliner(options, transform).apply();
 }
 
 const TYPE_DART_APP = 'application/dart';
 const TYPE_DART_COMPONENT = 'application/dart;component=1';
 const TYPE_JS = 'text/javascript';
 
 /// Internally adjusts urls in the html that we are about to inline.
@@ skipping 128 matching lines @@
 ];
 
 _getSpan(SourceFile file, AstNode node) => file.span(node.offset, node.end);
 
 const COMPONENT_WARNING =
     'More than one Dart script per HTML document is not supported, but in the '
     'near future Dartium will execute each tag as a separate isolate. If this '
     'code is meant to load definitions that are part of the same application '
     'you should switch it to use the "application/dart;component=1" mime-type '
     'instead.';