Fix hints reported by analyzer.

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 '../closure.dart';
-import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry;
 import '../compiler.dart';
-import '../constants/constant_system.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
-import '../io/source_information.dart';
 import '../js_backend/js_backend.dart';
 import '../resolution/tree_elements.dart';
 import '../tree/tree.dart' as ast;
 import '../types/types.dart';
-import '../universe/call_structure.dart' show CallStructure;
-import '../universe/use.dart' show TypeUse;
 import '../world.dart' show ClosedWorld;
 import 'jump_handler.dart';
 import 'locals_handler.dart';
 import 'nodes.dart';
-import 'ssa_branch_builder.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
 /// the current state of the graph being built.
 abstract class GraphBuilder {
   /// Holds the resulting SSA graph.
   final HGraph graph = new HGraph();
 
@@ skipping 301 matching lines @@
     if (unaliased is ResolutionTypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(ResolutionDynamicType type, GraphBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.helpers.DynamicRuntimeType;
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, closedWorld)));
   }
 }