Merge more Kernel infrastructure from kernel_sdk SDK fork.

runtime/lib/lib_prefix.dart

Index: runtime/lib/lib_prefix.dart
diff --git a/runtime/lib/lib_prefix.dart b/runtime/lib/lib_prefix.dart
index f457b020f8a722e43685eee8787832d7b1c31158..1b0e87d79dccbd1616246e72e96428379331a369 100644
--- a/runtime/lib/lib_prefix.dart
+++ b/runtime/lib/lib_prefix.dart
@@ -2,9 +2,6 @@
 // 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.
 
-import "dart:async";
-import "dart:isolate";
-
 // This type corresponds to the VM-internal class LibraryPrefix.
 class _LibraryPrefix {
   bool _load() native "LibraryPrefix_load";