Cache snapshots of (mostly) immutable transformer phases.

sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart

 // Copyright (c) 2014, 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.asset.transformer_isolate;
 
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:mirrors';
 
 import 'package:barback/barback.dart';
 
 import 'serialize.dart';
 
+/// The mirror system.
+///
+/// Cached to avoid re-instantiating each time a transformer is initialized.
+final _mirrors = currentMirrorSystem();
+
+/// The URI of this library.
+final _baseUri = _mirrors.findLibrary(
+    const Symbol('pub.asset.transformer_isolate')).uri;
+
 /// Sets up the initial communication with the host isolate.
 void loadTransformers(SendPort replyTo) {
   var port = new ReceivePort();
   replyTo.send(port.sendPort);
   port.listen((wrappedMessage) {
     // TODO(nweiz): When issue 19228 is fixed, spin up a separate isolate for
     // libraries loaded beyond the first so they can run in parallel.
     respond(wrappedMessage, (message) {
-      var library = Uri.parse(message['library']);
       var configuration = JSON.decode(message['configuration']);
       var mode = new BarbackMode(message['mode']);
-      return _initialize(library, configuration, mode).
+      return _initialize(message['library'], configuration, mode).
           map(serializeTransformerLike).toList();
     });
   });
 }
 
 /// Loads all the transformers and groups defined in [uri].
 ///
 /// Loads the library, finds any [Transformer] or [TransformerGroup] subclasses
 /// in it, instantiates them with [configuration] and [mode], and returns them.
-List _initialize(Uri uri, Map configuration, BarbackMode mode) {
-  var mirrors = currentMirrorSystem();
+List _initialize(String uri, Map configuration, BarbackMode mode) {
   var transformerClass = reflectClass(Transformer);
   var aggregateClass = _aggregateTransformerClass;
   var groupClass = reflectClass(TransformerGroup);
 
   var seen = new Set();
   var transformers = [];
 
   loadFromLibrary(library) {
     if (seen.contains(library)) return;
     seen.add(library);
@@ skipping 24 matching lines @@
         if (configuration.isNotEmpty) return null;
         return classMirror.newInstance(const Symbol('asPlugin'), []).reflectee;
       }
       if (constructor.parameters.length != 1) return null;
 
       return classMirror.newInstance(const Symbol('asPlugin'),
           [new BarbackSettings(configuration, mode)]).reflectee;
     }).where((classMirror) => classMirror != null));
   }
 
-  loadFromLibrary(mirrors.libraries[uri]);
+  loadFromLibrary(_mirrors.libraries[_baseUri.resolve(uri)]);
   return transformers;
 }
 
 // TODO(nweiz): clean this up when issue 13248 is fixed.
 MethodMirror _getConstructor(ClassMirror classMirror, String constructor) {
   var name = new Symbol("${MirrorSystem.getName(classMirror.simpleName)}"
       ".$constructor");
   var candidate = classMirror.declarations[name];
   if (candidate is MethodMirror && candidate.isConstructor) return candidate;
   return null;
 }
 
 // Older barbacks don't support [AggregateTransformer], and calling
 // [reflectClass] on an undefined class will throw an error, so we just define a
 // null getter for them.
 //# if barback >=0.14.1
 ClassMirror get _aggregateTransformerClass =>
     reflectClass(AggregateTransformer);
 //# else
 //>   ClassMirror get _aggregateTransformerClass => null;
 //# end