Fix all strong-mode warnings.

lib/src/transformer/base_transform.dart

Index: lib/src/transformer/base_transform.dart
diff --git a/lib/src/transformer/base_transform.dart b/lib/src/transformer/base_transform.dart
index fe3ab3b6cdd3cf0708a66cc5cef89dbb9a49948b..69492f0cc875db8051aceb0c97a96c9e5d435fb1 100644
--- a/lib/src/transformer/base_transform.dart
+++ b/lib/src/transformer/base_transform.dart
@@ -76,7 +76,7 @@ abstract class BaseTransform {
 /// [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;
@@ -92,8 +92,6 @@ abstract class BaseTransformController {
   /// 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