Support [Transform.hasInput] and [Transform.consumePrimary] in pub.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index e385ef34a2c0b47ffbaec04fa4c09887f1d8d266..b1a8eb3c30ac464bb47dbab39cc769f80d50a533 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -123,6 +123,13 @@ class ForeignTransform implements Transform {
     }).then(deserializeAsset);
   }
 
+  Future<bool> hasInput(AssetId id) {
+    return getInput(id).then((_) => true).catchError((error) {
+      if (error is AssetNotFoundException && error.id == id) return false;
+      throw error;
+    });
+  }
+
   Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
     if (encoding == null) encoding = UTF8;
     return getInput(id).then((input) => input.readAsString(encoding: encoding));
@@ -137,6 +144,10 @@ class ForeignTransform implements Transform {
       'output': serializeAsset(output)
     });
   }
+
+  void consumePrimary() {
+    _call(_port, {'type': 'consumePrimary'});
+  }
 }
 
 /// Returns the mirror for the root Object type.
@@ -536,6 +547,11 @@ Map _serializeTransform(Transform transform) {
         return null;
       }
 
+      if (message['type'] == 'consumePrimary') {
+        transform.consumePrimary();
+        return null;
+      }
+
       assert(message['type'] == 'log');
       var method;
       if (message['level'] == 'Info') {