Introduce polymer transformers (inlined code extraction, inlining html imports,

pkg/polymer/lib/src/transform/code_extractor.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 extracts inlined script code into separate assets. */
+library polymer.src.transformers;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import 'common.dart';
+
+/**
+ * Transformer that extracts Dart code inlined in HTML script tags and outputs a
+ * separate file for each.
+ */
+class InlineCodeExtractor extends Transformer {
+  Future<bool> isPrimary(Asset input) =>
+      new Future.value(input.id.extension == ".html");
+
+  Future apply(Transform transform) {
+    var inputId = transform.primaryId;
+    return getPrimaryContent(transform).then((content) {
+      var document = parseHtml(content, inputId.path, transform.logger);
+      int count = 0;
+      for (var tag in document.queryAll('script')) {
+        if (tag.attributes['type'] == 'application/dart' &&
+            !tag.attributes.containsKey('src')) {
+          // TODO(sigmund): should we automatically include a library directive
+          // if it doesn't have one?
+          var filename = path.basename(inputId.path);
+          tag.attributes['src'] = '$filename.$count.dart';
+          var textContent = tag.nodes.first;
+          var id = inputId.addExtension('.$count.dart');
+          transform.addOutput(new Asset.fromString(id, textContent.value));
+          textContent.remove();
+          count++;
+        }
+      }
+      transform.addOutput(new Asset.fromString(inputId,
+          count == 0 ? content : document.outerHtml));
+    });
+  }
+}