Improvements to the kernel tree shaker.

pkg/kernel/lib/target/vm.dart

Index: pkg/kernel/lib/target/vm.dart
diff --git a/pkg/kernel/lib/target/vm.dart b/pkg/kernel/lib/target/vm.dart
index 58c229dc14f26bf444fa29fba003d5a5de3d2320..c12403ec6384ba44464b0e0c2c02e6587ffc298a 100644
--- a/pkg/kernel/lib/target/vm.dart
+++ b/pkg/kernel/lib/target/vm.dart
@@ -12,6 +12,7 @@ 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.
@@ -57,15 +58,20 @@ class VmTarget extends Target {
     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.