Merge more Kernel infrastructure from kernel_sdk SDK fork.

sdk/lib/internal/internal.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library dart._internal;
 
 import 'dart:collection';
 
 import 'dart:core' hide Symbol;
 import 'dart:core' as core;
@@ skipping 42 matching lines @@
   /** The string that this is the code units of. */
   final String _string;
 
   CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 
   static String stringOf(CodeUnits u) => u._string;
 }
+
+/// Marks a function as an external implementation ("native" in the Dart VM).
+///
+/// Provides a backend-specific String that can be used to identify the
+/// function's implementation.
+class ExternalName {
+  final String name;
+  const ExternalName(this.name);
+}