Implement least upper bound.

sdk/lib/_internal/compiler/implementation/resolution/resolution.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 resolution;
 
 import 'dart:collection' show Queue;
 
 import '../dart2jslib.dart' hide Diagnostic;
 import '../dart_types.dart';
 import '../../compiler.dart' show Diagnostic;
 import '../tree/tree.dart';
 import '../elements/elements.dart';
 import '../elements/modelx.dart'
-    show FunctionElementX,
+    show BaseClassElementX,
+         FunctionElementX,
          ErroneousElementX,
          VariableElementX,
          FieldParameterElementX,
          VariableListElementX,
          FunctionSignatureX,
          LabelElementX,
          TargetElementX,
          MixinApplicationElementX,
          TypedefElementX,
          SynthesizedConstructorElementX;
 import '../util/util.dart';
 import '../scanner/scannerlib.dart' show PartialMetadataAnnotation;
 
 import 'secret_tree_element.dart' show getTreeElement, setTreeElement;
+import '../ordered_typeset.dart' show OrderedTypeSet, OrderedTypeSetBuilder;
 
 part 'members.dart';
 part 'scope.dart';