Only run [declareOutputs] once for each asset/transformer pair.

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.transformer;
 
 import 'dart:async';
 
@@ skipping 27 matching lines @@
     } 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' ||
-        (_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<bool> isPrimary(AssetId id) => new Future.value(
+      id.extension != '.dart' || (_files != null && !_files.contains(id.path)));
 
   Future apply(Transform transform) {
     return transform.primaryInput.readAsString().then((content) {
+      // Do a quick string check to determine if this is this file even
+      // plausibly might need to be transformed. If not, we can avoid an
+      // expensive parse.
+      if (!content.contains("@observable") && !content.contains("@published")) {
+        return;
+      }
+
       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(transform.primaryInput);
         return;
@@ skipping 352 matching lines @@
 
 Token _findFieldSeperator(Token token) {
   while (token != null) {
     if (token.type == TokenType.COMMA || token.type == TokenType.SEMICOLON) {
       break;
     }
     token = token.next;
   }
   return token;
 }