Don't expect String on read-errors.

tests/compiler/dart2js/memory_source_file_helper.dart

Index: tests/compiler/dart2js/memory_source_file_helper.dart
diff --git a/tests/compiler/dart2js/memory_source_file_helper.dart b/tests/compiler/dart2js/memory_source_file_helper.dart
index 8ebdf7d6763223ad1d2c4e5f63cbb651b049349b..135b025b1160a8732fac3907ff87a753a4b7e3fe 100644
--- a/tests/compiler/dart2js/memory_source_file_helper.dart
+++ b/tests/compiler/dart2js/memory_source_file_helper.dart
@@ -33,8 +33,9 @@ class MemorySourceFileProvider extends SourceFileProvider {
       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);