Fix a synchrony bug in barback.

pkg/barback/lib/src/phase_input.dart

Index: pkg/barback/lib/src/phase_input.dart
diff --git a/pkg/barback/lib/src/phase_input.dart b/pkg/barback/lib/src/phase_input.dart
index 10970221ee62e8b3e491ac19d37c1a0fa54f762a..552fa5daf9f5a7b1e536eaca2561db24d56cf1f9 100644
--- a/pkg/barback/lib/src/phase_input.dart
+++ b/pkg/barback/lib/src/phase_input.dart
@@ -275,14 +275,19 @@ class PhaseInput {
   /// Processes the transforms for this input.
   Future<Set<AssetNode>> process() {
     if (_adjustTransformersFuture == null) return _processTransforms();
-    return _waitForInputs().then((_) => _processTransforms());
+    return _waitForTransformers(() => _processTransforms());
   }
 
-  Future _waitForInputs() {
-    // Return a synchronous future so we can be sure [_adjustTransformers] isn't
-    // called between now and when the Future completes.
-    if (_adjustTransformersFuture == null) return new Future.sync(() {});
-    return _adjustTransformersFuture.then((_) => _waitForInputs());
+  /// Runs [callback] once all the transformers are adjusted correctly and the
+  /// input is ready to be processed.
+  ///
+  /// If the transformers are already properly adjusted, [callback] is called
+  /// synchronously to ensure that [_adjustTransformers] isn't called before the
+  /// callback.
+  Future _waitForTransformers(callback()) {
+    if (_adjustTransformersFuture == null) return new Future.sync(callback);
+    return _adjustTransformersFuture.then(
+        (_) => _waitForTransformers(callback));
   }
 
   /// Applies all currently wired up and dirty transforms.