Tree-shaker: drop imports/exports in a way that doesn't break the vm-service.

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 10227 matching lines @@
       if (imported.IsCoreLibrary()) {
         return true;
       }
     }
   }
   return false;
 }
 
 
 void Library::DropDependencies() const {
-  StorePointer(&raw_ptr()->imports_, Array::null());
-  StorePointer(&raw_ptr()->exports_, Array::null());
+  StorePointer(&raw_ptr()->imports_, Object::empty_array().raw());
+  StorePointer(&raw_ptr()->exports_, Object::empty_array().raw());
 }
 
 
 void Library::AddImport(const Namespace& ns) const {
   Array& imports = Array::Handle(this->imports());
   intptr_t capacity = imports.Length();
   if (num_imports() == capacity) {
     capacity = capacity + kImportsCapacityIncrement;
     imports = Array::Grow(imports, capacity);
     StorePointer(&raw_ptr()->imports_, imports.raw());
@@ skipping 12351 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart