Per isolate package root.

runtime/lib/isolate_patch.dart

 // Copyright (c) 2012, 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.
 
 import "dart:collection" show HashMap;
 
 patch class ReceivePort {
   /* patch */ factory ReceivePort() = _ReceivePortImpl;
 
   /* patch */ factory ReceivePort.fromRawReceivePort(RawReceivePort rawPort) =
@@ skipping 259 matching lines @@
         readyPort.close();
       }
       return new Future<Isolate>.error(e, st);
     };
   }
 
   /* patch */ static Future<Isolate> spawnUri(
       Uri uri, List<String> args, var message,
       { bool paused: false, Uri packageRoot }) {
     // `paused` isn't handled yet.
-    // `packageRoot` isn't handled yet.
-    if (packageRoot != null) throw new UnimplementedError("packageRoot");
     RawReceivePort readyPort;
     try {
       // The VM will invoke [_startIsolate] and not `main`.
       readyPort = new RawReceivePort();
-      _spawnUri(readyPort.sendPort, uri.toString(), args, message);
+      var packageRootString;
+      if (packageRoot != null) {
+        packageRootString = packageRoot.toString();
+      }
+      _spawnUri(
+          readyPort.sendPort, uri.toString(), args, message, packageRootString);
       Completer completer = new Completer<Isolate>.sync();
       readyPort.handler = (readyMessage) {
         readyPort.close();
         assert(readyMessage is List);
         assert(readyMessage.length == 2);
         SendPort controlPort = readyMessage[0];
         List capabilities = readyMessage[1];
         completer.complete(new Isolate(controlPort,
                                        pauseCapability: capabilities[0],
                                        terminateCapability: capabilities[1]));
@@ skipping 12 matching lines @@
   // These values need to be kept in sync with the class IsolateMessageHandler
   // in vm/isolate.cc.
   static const _PAUSE = 1;
   static const _RESUME = 2;
 
   static SendPort _spawnFunction(SendPort readyPort, Function topLevelFunction,
                                  var message)
       native "Isolate_spawnFunction";
 
   static SendPort _spawnUri(SendPort readyPort, String uri,
-                            List<String> args, var message)
+                            List<String> args, var message,
+                            String packageRoot)
       native "Isolate_spawnUri";
 
   static void _sendOOB(port, msg) native "Isolate_sendOOB";
 
   /* patch */ void _pause(Capability resumeCapability) {
     var msg = new List(4)
         ..[0] = 0  // Make room for OOM message type.
         ..[1] = _PAUSE
         ..[2] = pauseCapability
         ..[3] = resumeCapability;
@@ skipping 30 matching lines @@
   }
 
   /* patch */ void addErrorListener(SendPort port) {
     throw new UnsupportedError("addErrorListener");
   }
 
   /* patch */ void removeErrorListener(SendPort port) {
     throw new UnsupportedError("removeErrorListener");
   }
 }