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

pkg/compiler/lib/src/ssa/graph_builder.dart

 // Copyright (c) 2016, 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 '../common/codegen.dart' show CodegenRegistry;
 import '../compiler.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart' show Entity, Local;
 import '../elements/resolution_types.dart';
 import '../js_backend/js_backend.dart';
 import '../resolution/tree_elements.dart';
-import '../tree/tree.dart' as ast;
 import '../types/types.dart';
 import '../world.dart' show ClosedWorld;
 import 'jump_handler.dart';
 import 'locals_handler.dart';
 import 'nodes.dart';
 import 'type_builder.dart';
 
 /// Base class for objects that build up an SSA graph.
 ///
 /// This contains helpers for building the graph and tracking information about
@@ skipping 204 matching lines @@
     open(newBlock);
   }
 
   HInstruction callSetRuntimeTypeInfo(
       HInstruction typeInfo, HInstruction newObject);
 
   /// The element for which this SSA builder is being used.
   Element get targetElement;
   TypeBuilder get typeBuilder;
 }