Per isolate package root.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index e0194fc649faa75509ba57a9ee49cb089cf68774..1313609002de2d77afab21a30c262f499610bcf0 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -277,13 +277,14 @@ patch class Isolate {
       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 =
+          (packageRoot == null) ? null : packageRoot.toString();
+      _spawnUri(
+          readyPort.sendPort, uri.toString(), args, message, packageRootString);
       Completer completer = new Completer<Isolate>.sync();
       readyPort.handler = (readyMessage) {
         readyPort.close();
@@ -316,7 +317,8 @@ patch class Isolate {
       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";