Pass a package root to spawned isolates in pub.

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 configuration = JSON.decode(message['configuration']);
       var mode = new BarbackMode(message['mode']);
@@ skipping 45 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[_baseUri.resolve(uri)]);
+  var library = _mirrors.libraries[Uri.parse(uri)];
+
+  // This should only happen if something's wrong with the logic in pub itself.
+  // If it were user error, the entire isolate would fail to load.
+  if (library == null) throw "Couldn't find library at $uri.";
+
+  loadFromLibrary(library);
   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