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..66d9fea34fa520245436130f8341d5689e6b2351 100644
--- a/tests/utils/recursive_import_test.dart
+++ b/tests/utils/recursive_import_test.dart
@@ -69,7 +69,7 @@ main() {
     String source;
     if (uri.path.length > 100) {
       // Simulate an OS error.
-      throw 'Path length exceeded';
+      return new Future.error('Path length exceeded');
     } else if (uri.scheme == "main") {
       count++;
       source = RECURSIVE_MAIN;
@@ -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);
   }