Fix error message for library name mismatch.

tests/compiler/dart2js/mock_compiler.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 mock_compiler;
 
 import 'dart:collection';
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 250 matching lines @@
     // Do nothing. The mock core library is already handled in the constructor.
   }
 
   void importCoreLibrary(LibraryElement library) {
     scanner.importLibrary(library, coreLibrary, null);
   }
 
   // The mock library doesn't need any patches.
   Uri resolvePatchUri(String dartLibraryName) => null;
 
-  Script readScript(Uri uri, [ScriptTag node]) {
+  Script readScript(Uri uri, [Node node]) {
     SourceFile sourceFile = sourceFiles[uri.toString()];
     if (sourceFile == null) throw new ArgumentError(uri);
     return new Script(uri, sourceFile);
   }
 
   Element lookupElementIn(ScopeContainerElement container, name) {
     Element element = container.localLookup(name);
     return element != null
         ? element
         : new ErroneousElementX(null, null, name, container);
@@ skipping 47 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }