Add package implementation.

lib/src/package_root_resolver.dart

Index: lib/src/package_root_resolver.dart
diff --git a/lib/src/package_root_resolver.dart b/lib/src/package_root_resolver.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c9a06fff4f503c077dc3f1a380c03e6a192ec7fc
--- /dev/null
+++ b/lib/src/package_root_resolver.dart
@@ -0,0 +1,59 @@
+// Copyright (c) 2016, 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.
+
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+
+import 'async_package_resolver.dart';
+import 'package_resolver.dart';
+import 'sync_package_resolver.dart';
+import 'utils.dart';
+
+/// A package resolution strategy based on a package root URI.
+class PackageRootResolver implements SyncPackageResolver {
+  final packageConfigMap = null;
+  final packageConfigUri = null;
+
+  final Uri packageRoot;
+
+  PackageResolver get asAsync => new AsyncPackageResolver(this);
+
+  String get processArgument => "--package-root=$packageRoot";
+
+  PackageRootResolver(packageRoot)
+      : packageRoot = ensureTrailingSlash(asUri(packageRoot, "packageRoot"));
+
+  Uri resolveUri(packageUri) {
+    packageUri = asPackageUri(packageUri, "packageUri");
+
+    // Following [Isolate.resolvePackageUri], "package:foo" resolves to null.
+    if (packageUri.pathSegments.length == 1) return null;
+    return packageRoot.resolve(packageUri.path);
+  }
+
+  Uri urlFor(String package, [String path]) {
+    var result = packageRoot.resolve("$package/");
+    return path == null ? result : result.resolve(path);
+  }
+
+  Uri packageUriFor(url) {
+    var packageRootString = packageRoot.toString();
+    url = asUri(url, "url").toString();
+    if (!p.url.isWithin(packageRootString, url)) return null;
+
+    var relative = p.url.relative(url, from: packageRootString);
+    if (!relative.contains("/")) relative += "/";
+    return Uri.parse("package:$relative");
+  }
+
+  String packagePath(String package) {
+    if (packageRoot.scheme != 'file') return null;
+
+    var libLink = p.join(p.fromUri(packageRoot), package);
+    if (!new Link(libLink).existsSync()) return null;
+
+    return p.dirname(new Link(libLink).resolveSymbolicLinksSync());
+  }
+}