First pass at asynchronous input loading in dart2js.

tests/utils/recursive_import_test.dart

Index: tests/utils/recursive_import_test.dart
diff --git a/tests/utils/recursive_import_test.dart b/tests/utils/recursive_import_test.dart
index f827089997e55ff92b8ad35f1f7c2e6daf2b0313..7921c2805e632806fa380c51768f3eeac3573809 100644
--- a/tests/utils/recursive_import_test.dart
+++ b/tests/utils/recursive_import_test.dart
@@ -86,7 +86,7 @@ main() {
         source = "library lib${uri.path.replaceAll('/', '.')};";
       }
     } else {
-     throw "unexpected URI $uri";
+     return new Future.error("unexpected URI $uri");
     }
     return new Future.value(source);
   }