Adds @HtmlImport with both dynamic and transformer based implementations. Still need an HtmlInliner…

lib/html_import_annotation.dart

Index: lib/html_import_annotation.dart
diff --git a/lib/html_import_annotation.dart b/lib/html_import_annotation.dart
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+// Copyright (c) 2015, 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.
+library web_components.html_import_annotation;
+
+import 'dart:async';
+import 'dart:html';
+import 'package:initialize/initialize.dart';
+import 'src/normalize_path.dart';
+
+/// Annotation for a dart library which injects an html import into the
+/// current html document. The imported file must not contain any dart script
+/// tags, as they cannot be dynamically loaded.
+class HtmlImport implements Initializer<LibraryIdentifier> {
+  final String filePath;
+
+  const HtmlImport(this.filePath);
+
+  Future initialize(LibraryIdentifier library) {
+    var element = new LinkElement()
+      ..rel = 'import'
+      ..href = normalizeHtmlImportPath(filePath, library.package, library.path);
+    document.head.append(element);
+    var completer = new Completer();
+    var listeners = [
+      element.on['load'].listen((_) => completer.complete()),
+      element.on['error'].listen((_) {
+        print('Error loading html import from path `$filePath`');
+        completer.complete();
+      }),
+    ];
+    return completer.future.then((_) {
+      listeners.forEach((listener) => listener.cancel());
+    });
+  }
+}