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

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;
+    seen.add(id);
+    return getPrimaryContent(transform).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') {
+            tag.remove();
+          }
+        }
+        var fragment = new DocumentFragment()..nodes.addAll(elements);
+        document.body.insertBefore(fragment,
+            //TODO(jmesserly): add Node.firstChild to html5lib
+            document.body.nodes.length == 0 ? null : document.body.nodes[0]);
+        transform.addOutput(new Asset.fromString(id, document.outerHtml));
+      });
+    });
+  }
+
+  /**
+   * Visits imports in [document] and add their polymer-element definitions to
+   * [elements], unless they have already been [seen]. Elements are added in the
+   * order they appear, transitive imports are added first.
+   */
+  Future<bool> _visitImports(Document document, AssetId sourceId,
+      Transform transform, Set<AssetId> seen, List<Node> elements) {
+    var importIds = [];
+    bool hasImports = false;
+    for (var tag in document.queryAll('link')) {
+      if (tag.attributes['rel'] != 'import') continue;
+      var href = tag.attributes['href'];
+      var id = resolve(sourceId, href, transform.logger, tag.sourceSpan);
+      hasImports = true;
+      if (id == null || seen.contains(id)) continue;
+      importIds.add(id);
+    }
+
+    if (importIds.isEmpty) return new Future.value(hasImports);
+
+    // Note: we need to preserve the import order in the generated output.
+    return Future.forEach(importIds, (id) {
+      if (seen.contains(id)) return new Future.value(null);
+      seen.add(id);
+      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)
+        .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')));
+        });
+  }
+}