Importing unittest disables tree shaking

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked;
 
   JavaScriptItemCompilationContext()
@@ skipping 1412 matching lines @@
   ClassElement get constMapImplementation => constMapLiteralClass;
   ClassElement get typeImplementation => typeLiteralClass;
   ClassElement get boolImplementation => jsBoolClass;
   ClassElement get nullImplementation => jsNullClass;
 
   void registerStaticUse(Element element, Enqueuer enqueuer) {
     if (element == disableTreeShakingMarker) {
       compiler.disableTypeInferenceForMirrors = true;
       isTreeShakingDisabled = true;
       enqueuer.enqueueEverything();
+      throw 'Unexpected disable tree shaking';
     } else if (element == preserveNamesMarker) {
       if (mustPreserveNames) return;
       mustPreserveNames = true;
       compiler.log('Preserving names.');
     } else if (element == preserveMetadataMarker) {
       if (mustRetainMetadata) return;
       compiler.log('Retaining metadata.');
       mustRetainMetadata = true;
       for (LibraryElement library in compiler.libraries.values) {
         if (retainMetadataOf(library)) {
@@ skipping 199 matching lines @@
     copy(constant.values);
     copy(constant.protoValue);
     copy(constant);
   }
 
   void visitConstructed(ConstructedConstant constant) {
     copy(constant.fields);
     copy(constant);
   }
 }