Fix all strong-mode warnings.

lib/src/transformer/base_transform.dart

 // 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 barback.transformer.base_transform;
 
 import 'dart:async';
 
 import '../asset/asset_id.dart';
 import '../graph/transform_node.dart';
@@ skipping 58 matching lines @@
     _consumedPrimaries.add(id);
   }
 }
 
 /// The base class for controllers of subclasses of [BaseTransform].
 ///
 /// Controllers are used so that [TransformNode]s can get values from a
 /// [BaseTransform] without exposing getters in the public API.
 abstract class BaseTransformController {
   /// The [BaseTransform] controlled by this controller.
-  final BaseTransform transform;
+  BaseTransform get transform;
 
   /// The ids of primary inputs that should be consumed.
   Set<AssetId> get consumedPrimaries => transform._consumedPrimaries;
 
   /// Whether the transform logged an error.
   bool get loggedError => transform._loggedError;
 
   /// The stream of log entries emitted by the transformer during a run.
   Stream<LogEntry> get onLog => transform._onLogController.stream;
 
   /// Whether the transform's input or id stream has been closed.
   ///
   /// See also [done].
   bool get isDone;
 
-  BaseTransformController(this.transform);
-
   /// Mark this transform as finished emitting new inputs or input ids.
   ///
   /// This is distinct from [cancel] in that it *doesn't* indicate that the
   /// transform is finished being used entirely. The transformer may still log
   /// messages and load secondary inputs. This just indicates that all the
   /// primary inputs are accounted for.
   void done();
 
   /// Mark this transform as canceled.
   ///
   /// This will close any streams and release any resources that were allocated
   /// for the duration of the transformation. Unlike [done], this indicates that
   /// the transformation is no longer relevant; either it has returned, or
   /// something external has preemptively invalidated its results.
   void cancel() {
     done();
     transform._onLogController.close();
   }
 }