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

BUG=http://dartbug.com/26550
R=hausner@google.com

Review URL: https://codereview.chromium.org/2014913002 .
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index de74eb4..0d5ca5f 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -10245,8 +10245,8 @@
 
 
 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());
 }
 
 
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 1543db6..b245a94 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1300,7 +1300,7 @@
                      kAsReference);
   if (kind == Snapshot::kAppNoJIT) {
     prefix.StorePointer(&prefix.raw_ptr()->imports_,
-                        Array::null());
+                        Object::empty_array().raw());
   }
   prefix.StorePointer(&prefix.raw_ptr()->dependent_code_,
                       Array::null());