Don't expect String on read-errors.

sdk/lib/_internal/compiler/implementation/apiimpl.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 leg_apiimpl;
 
 import 'dart:async';
 
 import '../compiler.dart' as api;
 import 'dart2jslib.dart' as leg;
@@ skipping 146 matching lines @@
    */
   Future<leg.Script> readScript(Uri readableUri,
                                 [elements.Element element, tree.Node node]) {
     if (!readableUri.isAbsolute) {
       internalError('Relative uri $readableUri provided to readScript(Uri)',
                     node: node);
     }
 
     // TODO(johnniwinther): Add [:report(..., {Element element}):] to
     // report methods in Compiler.
-    void reportReadError(String exception) {
+    void reportReadError(exception) {
       withCurrentElement(element, () {
         reportError(node,
                     leg.MessageKind.READ_SCRIPT_ERROR,
                     {'uri': readableUri, 'exception': exception});
       });
     }
 
     Uri resourceUri = translateUri(readableUri, node);
     // TODO(johnniwinther): Wrap the result from [provider] in a specialized
     // [Future] to ensure that we never execute an asynchronous action without setting
@@ skipping 110 matching lines @@
       handler(translateUri(span.uri, null), span.begin, span.end,
               message, kind);
     }
   }
 
   bool get isMockCompilation {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') != -1);
   }
 }