Revert "Improvements to the kernel tree shaker."

pkg/kernel/lib/target/vm.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.
 library kernel.target.vm;
 
 import '../ast.dart';
 import '../core_types.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
 import '../transformations/insert_covariance_checks.dart';
 import '../transformations/insert_type_checks.dart';
 import '../transformations/mixin_full_resolution.dart' as mix;
 import '../transformations/sanitize_for_vm.dart';
 import '../transformations/setup_builtin_library.dart' as setup_builtin_library;
-import '../transformations/treeshaker.dart';
 import 'targets.dart';
 
 /// Specializes the kernel IR to the Dart VM.
 class VmTarget extends Target {
   final TargetFlags flags;
 
   VmTarget(this.flags);
 
   bool get strongMode => flags.strongMode;
 
@@ skipping 25 matching lines @@
         'dart:_vmservice',
         'dart:_builtin',
         'dart:nativewrappers',
         'dart:io',
       ];
 
   void transformProgram(Program program) {
     var mixins = new mix.MixinFullResolution();
     mixins.transform(program);
 
-    var hierarchy = mixins.hierarchy;
-    var coreTypes = new CoreTypes(program);
-
     if (strongMode) {
+      var hierarchy = mixins.hierarchy;
+      var coreTypes = new CoreTypes(program);
       new InsertTypeChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
       new InsertCovarianceChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
     }
 
-    new TreeShaker(program,
-            hierarchy: hierarchy, coreTypes: coreTypes, strongMode: strongMode)
-        .transform(program);
-
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.
     setup_builtin_library.transformProgram(program);
 
     if (strongMode) {
       new Erasure().transform(program);
     }
 
     new SanitizeForVM().transform(program);
   }
 }