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 db0eff38a40fc800683892881a3ce274294e7bff..46e22f572caa6cf10f7e5a570e55ab923c269ede 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);
   }