Make backend.setAnnotations (mostly) handle LibraryEntities.

pkg/compiler/lib/src/kernel/world_builder.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.
 
 library dart2js.kernel.world_builder;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/resolution.dart';
@@ skipping 438 matching lines @@
     for (ir.VariableDeclaration variable in sortedNamedParameters) {
       namedParameters.add(variable.name);
       namedParameterTypes.add(getDartType(variable.type));
     }
     return new FunctionType(returnType, parameterTypes, optionalParameterTypes,
         namedParameters, namedParameterTypes);
   }
 
   LibraryEntity getLibrary(ir.Library node) => _getLibrary(node);
 
+  ir.Library getKernelLibrary(KLibrary entity) =>
+      _libraryEnvs[entity.libraryIndex].library;
+
+  ir.Class getKernelClass(KClass entity) => _classEnvs[entity.classIndex].cls;
+
   @override
   Local getLocalFunction(ir.TreeNode node) => _getLocal(node);
 
   @override
   ClassEntity getClass(ir.Class node) => _getClass(node);
 
   @override
   FieldEntity getField(ir.Field node) => _getField(node);
 
   TypeVariableEntity getTypeVariable(ir.TypeParameter node) =>
@@ skipping 532 matching lines @@
   }
 
   InterfaceType getMixinTypeForClass(KClass cls) {
     KClassEnv env = builder._classEnvs[cls.classIndex];
     ir.Supertype mixedInType = env.cls.mixedInType;
     if (mixedInType == null) return null;
     return builder.createInterfaceType(
         mixedInType.classNode, mixedInType.typeArguments);
   }
 }