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/test/transform/common/read_file.dart

@@ -1,7 +1,11 @@
 library angular2.test.transform.common.read_file;
 
+import 'dart:async';
 import 'dart:io';
 
+import 'package:angular2/src/transform/common/asset_reader.dart';
+import 'package:barback/barback.dart';
+
 /// Smooths over differences in CWD between IDEs and running tests in Travis.
 String readFile(String path) {
   for (var myPath in [path, 'test/transform/${path}']) {
@@ -12,3 +16,17 @@ String readFile(String path) {
   }
   return null;
 }
+
+class TestAssetReader implements AssetReader {
+  Future<String> readAsString(AssetId id, {Encoding encoding}) =>
+      new Future.value(readFile(id.path));
+
+  Future<bool> hasInput(AssetId id) {
+    var exists = false;
+    for (var myPath in [id.path, 'test/transform/${id.path}']) {
+      var file = new File(myPath);
+      exists = exists || file.existsSync();
+    }
+    return new Future.value(exists);
+  }
+}