Deal with extension loading from packages.

runtime/bin/builtin.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.
 
 library builtin;
 import 'dart:uri';
 
 // Corelib 'print' implementation.
 void _print(arg) {
   _Logger._printString(arg.toString());
@@ skipping 53 matching lines @@
   _entrypoint = base.resolve(scriptName);
   _logResolution("# Resolved script to: $_entrypoint");
 
   return _entrypoint.toString();
 }
 
 String _resolveUri(String base, String userString) {
   var baseUri = new Uri.fromString(base);
   _logResolution("# Resolving: $userString from $base");
 
-  // Relative URIs with scheme dart-ext should be resolved as if with no scheme.
   var uri = new Uri.fromString(userString);
   var resolved;
   if ('dart-ext' == uri.scheme) {
+    // Relative URIs with scheme dart-ext should be resolved as if with no
+    // scheme.
     resolved = baseUri.resolve(uri.path);
-    resolved = new Uri.fromComponents(scheme: "dart-ext", path: resolved.path);
+    var path = resolved.path;
+    if (resolved.scheme == 'package') {
+      // If we are resolving relative to a package URI we go directly to the
+      // file path and keep the dart-ext scheme. Otherwise, we will lose the
+      // package URI path part.
+      path = _filePathFromPackageUri(resolved);
+    }
+    resolved = new Uri.fromComponents(scheme: "dart-ext", path: path);
   } else {
     resolved = baseUri.resolve(userString);
   }
   _logResolution("# Resolved to: $resolved");
   return resolved.toString();
 }
 
 
 String _filePathFromUri(String userUri, bool isWindows) {
   var uri = new Uri.fromString(userUri);
@@ skipping 59 matching lines @@
   var path;
   if (_packageRoot != null) {
     path = "${_packageRoot}${uri.path}";
   } else {
     path = _entrypoint.resolve('packages/${uri.path}').path;
   }
 
   _logResolution("# Package: $path");
   return path;
 }