Make Transform a little more pleasant to use:

pkg/polymer/lib/src/transform/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.transform.import_inliner;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:html5lib/dom.dart' show Document, Node, DocumentFragment;
 import 'common.dart';
 
 /** Recursively inlines polymer-element definitions from html imports. */
 // TODO(sigmund): make sure we match semantics of html-imports for tags other
 // than polymer-element (see dartbug.com/12613).
 class ImportedElementInliner extends Transformer {
   Future<bool> isPrimary(Asset input) =>
       new Future.value(input.id.extension == ".html");
 
   Future apply(Transform transform) {
     var seen = new Set<AssetId>();
     var elements = [];
-    var id = transform.primaryId;
+    var id = transform.primaryInput.id;
     seen.add(id);
-    return getPrimaryContent(transform).then((content) {
+    return transform.primaryInput.readAsString().then((content) {
       var document = parseHtml(content, id.path, transform.logger);
       var future = _visitImports(document, id, transform, seen, elements);
       return future.then((importsFound) {
         if (!importsFound) {
           transform.addOutput(new Asset.fromString(id, content));
           return;
         }
 
         for (var tag in document.queryAll('link')) {
           if (tag.attributes['rel'] == 'import') {
@@ skipping 36 matching lines @@
       return _collectPolymerElements(id, transform, seen, elements);
     }).then((_) => true);
   }
 
   /**
    * Loads an asset identified by [id], visits its imports and collects it's
    * polymer-element definitions.
    */
   Future _collectPolymerElements(
       AssetId id, Transform transform, Set<AssetId> seen, List elements) {
-    return getContent(transform, id)
+    return transform.readInputAsString(id)
         .then((content) => parseHtml(
               content, id.path, transform.logger, checkDocType: false))
         .then((document) {
           return _visitImports(document, id, transform, seen, elements)
             .then((_) => elements.addAll(document.queryAll('polymer-element')));
         });
   }
 }