Ensure we run the observable transform on PolymerElement (fixes issue 14942)

pkg/observe/lib/transformer.dart

Index: pkg/observe/lib/transformer.dart
diff --git a/pkg/observe/lib/transform.dart b/pkg/observe/lib/transformer.dart
similarity index 94%
copy from pkg/observe/lib/transform.dart
copy to pkg/observe/lib/transformer.dart
index 5e261c02b49b641fe30f36867373bd9a9ca3a9c5..3e6526baea64f02f02eb1d75284e188f4499d673 100644
--- a/pkg/observe/lib/transform.dart
+++ b/pkg/observe/lib/transformer.dart
@@ -6,7 +6,7 @@
  * Code transform for @observable. The core transformation is relatively
  * straightforward, and essentially like an editor refactoring.
  */
-library observe.transform;
+library observe.transformer;
 
 import 'dart:async';
 
@@ -28,8 +28,33 @@ import 'package:source_maps/span.dart' show SourceFile;
  */
 class ObservableTransformer extends Transformer {
 
+  final List<String> _files;
+  ObservableTransformer();
+  ObservableTransformer.asPlugin(BarbackSettings settings)
+      : _files = _readFiles(settings.configuration['files']);
+
+  static List<String> _readFiles(value) {
+    if (value == null) return null;
+    var files = [];
+    bool error;
+    if (value is List) {
+      files = value;
+      error = value.any((e) => e is! String);
+    } else if (value is String) {
+      files = [value];
+      error = false;
+    } else {
+      error = true;
+    }
+    if (error) print('Invalid value for "files" in the observe transformer.');
+    return files;
+  }
+
   Future<bool> isPrimary(Asset input) {
-    if (input.id.extension != '.dart') return new Future.value(false);
+    if (input.id.extension != '.dart' ||
+        (_files != null && !_files.contains(input.id.path))) {
+      return new Future.value(false);
+    }
     // Note: technically we should parse the file to find accurately the
     // observable annotation, but that seems expensive. It would require almost
     // as much work as applying the transform. We rather have some false