Be smarter about when to disable global type inference.

dart/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 1115 matching lines @@
     LibraryElement library = compiler.libraries['dart:_js_mirrors'];
     disableTreeShakingMarker =
         library.find(const SourceString('disableTreeShaking'));
     preserveNamesMarker =
         library.find(const SourceString('preserveNames'));
  }
 
   void registerStaticUse(Element element, Enqueuer enqueuer) {
     if (element == disableTreeShakingMarker) {
       enqueuer.enqueueEverything();
+      compiler.disableTypeInferenceForMirrors = true;
     } else if (element == preserveNamesMarker) {
     }
   }
 }