Make Transform a little more pleasant to use:

pkg/observe/lib/transform.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;
 
 import 'dart:async';
 import 'package:path/path.dart' as path;
 import 'package:analyzer_experimental/src/generated/ast.dart';
 import 'package:analyzer_experimental/src/generated/error.dart';
 import 'package:analyzer_experimental/src/generated/parser.dart';
 import 'package:analyzer_experimental/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. 
+ * implementation based on change notifications.
  *
  * The transformation adds hooks for field setters and notifies the observation
  * system of the change.
  */
 class ObservableTransformer extends Transformer {
 
   Future<bool> isPrimary(Asset input) {
     if (input.id.extension != '.dart') 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"));
   }
 
   Future apply(Transform transform) {
-    return transform.primaryInput
-        .then((input) => input.readAsString().then((content) {
-      var id = transform.primaryId;
+    return transform.primaryInput.readAsString().then((content) {
+      var id = transform.primaryInput.id;
       // TODO(sigmund): improve how we compute this url
       var url = id.path.startsWith('lib/')
             ? 'package:${id.package}/${id.path.substring(4)}' : id.path;
       var sourceFile = new SourceFile.text(url, content);
       var transaction = _transformCompilationUnit(
           content, sourceFile, transform.logger);
       if (!transaction.hasEdits) {
         transform.addOutput(input);
         return;
       }
       var printer = transaction.commit();
       // TODO(sigmund): emit source maps when barback supports it (see
       // dartbug.com/12340)
       printer.build(url);
       transform.addOutput(new Asset.fromString(id, printer.text));
-    }));
+    });
   }
 }
 
 TextEditTransaction _transformCompilationUnit(
     String inputCode, SourceFile sourceFile, TransformLogger logger) {
   var unit = _parseCompilationUnit(inputCode);
   return transformObservables(unit, sourceFile, inputCode, logger);
 }
 
 // TODO(sigmund): make this private. This is currently public so it can be used
@@ skipping 259 matching lines @@
 $type __\$$name$initializer;
 $type get $name => __\$$name;
 set $name($type value) {
   __\$$name = notifyPropertyChange(const Symbol('$name'), __\$$name, value);
 }
 '''.replaceAll('\n', '\n$indent'));
   }
 
   code.edit(begin, end, '$replace');
 }