Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/inferrer/builder_kernel.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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../compiler.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../kernel/kernel.dart';
 import '../ssa/kernel_ast_adapter.dart';
 import '../tree/tree.dart' as ast;
-import '../types/types.dart' show TypeMask;
 import '../universe/side_effects.dart' show SideEffects;
 import 'inferrer_engine.dart';
 import 'locals_handler.dart';
 import 'type_graph_nodes.dart';
 import 'type_system.dart';
 
 /// [KernelTypeGraphBuilder] constructs a type-inference graph for a particular
 /// element.
 ///
 /// Calling [run] will start the work of visiting the body of the code to
@@ skipping 138 matching lines @@
           : types.simplifyPhi(null, null, elementType);
       TypeInformation containerType =
           listLiteral.isConst ? types.constListType : types.growableListType;
       // TODO(efortuna): Change signature of allocateList and the rest of
       // type_system to deal with Kernel elements.
       return types.allocateList(containerType, astAdapter.getNode(listLiteral),
           outermostElement, elementType, length);
     });
   }
 }