pkg/compiler: fix imports

pkg/compiler/lib/src/serialization/constant_serialization.dart

 // Copyright (c) 2015, 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.serialization.constants;
 
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart' show FieldElement;
 import '../resolution/operators.dart';
 import '../universe/call_structure.dart' show CallStructure;
+import 'keys.dart';
 import 'serialization.dart';
-import 'keys.dart';
 
 /// Visitor that serializes a [ConstantExpression] by encoding it into an
 /// [ObjectEncoder].
 ///
 /// This class is called from the [Serializer] when a [ConstantExpression] needs
 /// serialization. The [ObjectEncoder] ensures that any [Element], [DartType],
 /// and other [ConstantExpression] that the serialized [ConstantExpression]
 /// depends upon are also serialized.
 class ConstantSerializer
     extends ConstantExpressionVisitor<dynamic, ObjectEncoder> {
@@ skipping 368 matching lines @@
             readFields(), readConstructorInvocation());
       case ConstantConstructorKind.REDIRECTING_GENERATIVE:
         return new RedirectingGenerativeConstantConstructor(
             readDefaults(), readConstructorInvocation());
       case ConstantConstructorKind.REDIRECTING_FACTORY:
         return new RedirectingFactoryConstantConstructor(
             readConstructorInvocation());
     }
   }
 }