Assert that we don't mix K and J elements

pkg/compiler/lib/src/kernel/kelements.dart

 // Copyright (c) 2017, 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.
 
 /// Entity model for elements derived from Kernel IR.
 
 import '../elements/entities.dart';
 import '../elements/names.dart';
 import '../elements/types.dart';
 import 'elements.dart';
 
-const String kElementPrefix = 'k';
+const String kElementPrefix = 'k:';
 
 class KLibrary implements IndexedLibrary {
   /// Library index used for fast lookup in [KernelWorldBuilder].
   final int libraryIndex;
   final String name;
   final Uri canonicalUri;
 
   KLibrary(this.libraryIndex, this.name, this.canonicalUri);
 
   String toString() => '${kElementPrefix}library($name)';
@@ skipping 223 matching lines @@
   final MemberEntity memberContext;
   final Entity executableContext;
   final FunctionType functionType;
 
   KLocalFunction(
       this.name, this.memberContext, this.executableContext, this.functionType);
 
   String toString() =>
       '${kElementPrefix}local_function(${memberContext.name}.${name ?? '<anonymous>'})';
 }