Don't expect String on read-errors.

tests/compiler/dart2js/memory_source_file_helper.dart

 // Copyright (c) 2013, 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 dart2js.test.memory_source_file_helper;
 
 import 'dart:async' show Future;
 import 'dart:io';
 export 'dart:io' show Platform;
 
@@ skipping 15 matching lines @@
 class MemorySourceFileProvider extends SourceFileProvider {
   final Map<String, String> memorySourceFiles;
 
   MemorySourceFileProvider(Map<String, String> this.memorySourceFiles);
 
   Future<String> readStringFromUri(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.readStringFromUri(resourceUri);
     }
     String source = memorySourceFiles[resourceUri.path];
-    // TODO(ahe): Return new Future.error(...) ?
-    if (source == null) return new Future.error('No such file $resourceUri');
+    if (source == null) {
+      return new Future.error(new Exception('No such file $resourceUri'));

[ahe, 2013/09/10 10:50:28]

Why do you need to wrap this in an exception?

[Johnni Winther, 2013/09/10 11:41:53]

I don't need to, but I found the type error in reportReadError when an imported
file could not be found by the file system. Wrapping the string in an exception
mimics the error from the file system and makes this behaviour testable in
missing_file_test.

+    }
     String resourceName = '$resourceUri';
     this.sourceFiles[resourceName] = new SourceFile(resourceName, source);
     return new Future.value(source);
   }
 }