feat(dart/transform): Add a .ng_deps.dart file parser.

Create a common, reusable `.ng_deps.dart` file parser. All future transformer
phases build on the information in `.ng_deps.dart` files.

modules/angular2/src/transform/common/asset_reader.dart

@@ -0,0 +1,26 @@
+library angular2.src.transform.common.asset_reader;
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:barback/barback.dart';
+
+abstract class AssetReader {
+  Future<String> readAsString(AssetId id, {Encoding encoding});
+  Future<bool> hasInput(AssetId id);
+
+  /// Creates an [AssetReader] using the `transform`, which should be a
+  /// [Transform] or [AggregateTransform].
+  factory AssetReader.fromTransform(dynamic transform) =>
+      new _TransformAssetReader(transform);
+}
+
+class _TransformAssetReader implements AssetReader {
+  final dynamic t;
+  _TransformAssetReader(this.t);
+
+  Future<String> readAsString(AssetId id, {Encoding encoding}) =>
+      t.readInputAsString(id, encoding: encoding);
+
+  Future<bool> hasInput(AssetId id) => t.hasInput(id);
+}