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

lib/src/normalize_path.dart

+// 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.src.normalizePath;
+
+import 'package:path/path.dart' as path;
+
+String normalizeHtmlImportPath(
+    String filePath, String dartFilePackage, String dartFilePath) {
+  // If they already supplied a packages path, just return that.
+  if (filePath.startsWith('package:')) {
+    return filePath.replaceFirst('package:', 'packages/');
+  }
+
+  var dartFileDir = path.url.dirname(dartFilePath);
+  var segments = path.url.split(dartFileDir);
+  // The dartFileDir without the leading dir (web, lib, test, etc).
+  var dartFileSubDir = path.url.joinAll(segments.getRange(1, segments.length));
+
+  // Relative paths have no package supplied.
+  if (dartFilePackage == null) {
+    return path.url.normalize(path.url.join(dartFileSubDir, filePath));
+  }
+
+  // Only option left is a packages/ path.
+  return path.url.normalize(
+      path.url.join('packages/', dartFilePackage, dartFileSubDir, filePath));
+}