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

sdk/lib/_internal/pub/lib/src/barback.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.
 
 library pub.barback;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
@@ skipping 13 matching lines @@
 ///
 /// This includes a string literal of Dart code ([_TRANSFORMER_ISOLATE] in
 /// load_transformers.dart). That code imports "package:barback/barback.dart".
 /// This string is included in the SDK, but that import is resolved using the
 /// application’s version of Barback. That means it must tightly control which
 /// version of Barback the application is using so that it's one that pub
 /// supports.
 ///
 /// Whenever a new non-patch version of barback is published, this *must* be
 /// incremented to synchronize with that.
-final supportedVersions = new VersionConstraint.parse(">=0.11.0 <0.13.0");
+final supportedVersions = new VersionConstraint.parse(">=0.13.0-dev <0.14.0");
 
 /// A list of the names of all built-in transformers that pub exposes.
 const _BUILT_IN_TRANSFORMERS = const ['\$dart2js'];
 
 /// An identifier for a transformer and the configuration that will be passed to
 /// it.
 ///
 /// It's possible that the library identified by [this] defines multiple
 /// transformers. If so, [configuration] will be passed to all of them.
 class TransformerId {
@@ skipping 199 matching lines @@
   if (parts.length <= index + 1) {
     throw new FormatException(
         'Invalid URL path "${url.path}". Expected package name '
         'after "packages".');
   }
 
   var package = parts[index + 1];
   var assetPath = path.url.join("lib", path.url.joinAll(parts.skip(index + 2)));
   return new AssetId(package, assetPath);
 }