Deal with extension loading from packages.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 4aea641be59f180dc3d26382e7b95bd9bc49d4a7..f84a50fd9d38cbdc17d91a0a58bd71961a6f56c8 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -71,12 +71,20 @@ 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);
   }