dart2js: add support for configuration-specific imports.

tests/language/config_import_lib1b.dart

Index: tests/language/config_import_lib1b.dart
diff --git a/tests/language/deferred_call_empty_before_load_lib.dart b/tests/language/config_import_lib1b.dart
similarity index 90%
copy from tests/language/deferred_call_empty_before_load_lib.dart
copy to tests/language/config_import_lib1b.dart
index 32dd3765f615a60ec0fec480b33f863eca4162ab..59212de47b5ca40bedac0e25318c9608d7bc51a5 100644
--- a/tests/language/deferred_call_empty_before_load_lib.dart
+++ b/tests/language/config_import_lib1b.dart
@@ -2,4 +2,4 @@
 // 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.
 
-thefun() {}
+String lib1() => "b";