Revert "Per isolate package root."

sdk/lib/isolate/isolate.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.
 
 /**
  * Concurrent programming using _isolates_:
  * independent workers that are similar to threads
  * but don't share memory,
  * communicating only via messages.
  */
@@ skipping 109 matching lines @@
    * The `packageRoot` URI must be a "file" or "http"/"https" URI that specifies
    * a directory. If it doesn't end in a slash, one will be added before
    * using the URI, and any query or fragment parts are ignored.
    * Package imports (like "package:foo/bar.dart") in the new isolate are
    * resolved against this location, as by
    * `packageRoot.resolve("foo/bar.dart")`.
    * This includes the main entry [uri] if it happens to be a package-URL.
    * If [packageRoot] is omitted, it defaults to the same URI that
    * the current isolate is using.
    *
-   * WARNING: The [packageRoot] parameter is not implemented on all
-   * platforms yet.
+   * WARNING: The [packageRoot] parameter is not implemented on any
+   * platform yet.
    *
    * If the [paused] parameter is set to `true`,
    * the isolate will start up in a paused state,
    * as if by an initial call of `isolate.pause(isolate.pauseCapability)`.
    * This allows setting up error or exit listeners on the isolate
    * before it starts running.
    * To resume the isolate, call `isolate.resume(isolate.pauseCapability)`.
    *
    * WARNING: The `pause` parameter is not implemented on all platforms yet.
    *
@@ skipping 407 matching lines @@
       : _description = description,
         stackTrace = new _RemoteStackTrace(stackDescription);
   String toString() => _description;
 }
 
 class _RemoteStackTrace implements StackTrace {
   String _trace;
   _RemoteStackTrace(this._trace);
   String toString() => _trace;
 }