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

pkg/observe/lib/transformer.dart

 // Copyright (c) 2013, 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.
 
 /**
  * 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';
 
 import 'package:analyzer/src/generated/java_core.dart' show CharSequence;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/parser.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:barback/barback.dart';
 import 'package:source_maps/refactor.dart';
 import 'package:source_maps/span.dart' show SourceFile;
 
 /**
  * A [Transformer] that replaces observables based on dirty-checking with an
  * implementation based on change notifications.
  *
  * The transformation adds hooks for field setters and notifies the observation
  * system of the change.
  */
 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
     // positives here, and then generate no outputs when we apply this
     // transform.
     return input.readAsString().then(
       (c) => c.contains("@observable") || c.contains("@published"));
   }
 
   Future apply(Transform transform) {
@@ skipping 363 matching lines @@
 
 Token _findFieldSeperator(Token token) {
   while (token != null) {
     if (token.type == TokenType.COMMA || token.type == TokenType.SEMICOLON) {
       break;
     }
     token = token.next;
   }
   return token;
 }