Fix observable transform bug introduced in a recent change, add regression tests

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;
 
@@ skipping 30 matching lines @@
   Future apply(Transform transform) {
     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);
+        transform.addOutput(transform.primaryInput);
         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));
     });
   }
 }
@@ 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');
 }