Expose aggregate transformers in pub.

dart/sdk/lib/_internal/pub/asset/dart/serialize/get_input_transform.dart

Index: dart/sdk/lib/_internal/pub/asset/dart/serialize/get_input_transform.dart
diff --git a/dart/sdk/lib/_internal/pub/asset/dart/serialize/get_input_transform.dart b/dart/sdk/lib/_internal/pub/asset/dart/serialize/get_input_transform.dart
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index 0000000000000000000000000000000000000000..9ab6540b3f9b0c3c32de42adb31256fc9bf2123b
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+++ b/dart/sdk/lib/_internal/pub/asset/dart/serialize/get_input_transform.dart
@@ -0,0 +1,34 @@
+// Copyright (c) 2014, 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 pub.asset.serialize.get_input_transform;
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:barback/barback.dart';
+
+import '../utils.dart';
+
+/// A mixin for transforms that support [getInput] and the associated suite of
+/// methods.
+abstract class GetInputTransform {
+  Future<Asset> getInput(AssetId id);
+
+  Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
+    if (encoding == null) encoding = UTF8;
+    return getInput(id).then((input) =>
+        input.readAsString(encoding: encoding));
+  }
+
+  Stream<List<int>> readInput(AssetId id) =>
+      futureStream(getInput(id).then((input) => input.read()));
+
+  Future<bool> hasInput(AssetId id) {
+    return getInput(id).then((_) => true).catchError((error) {
+      if (error is AssetNotFoundException && error.id == id) return false;
+      throw error;
+    });
+  }
+}