[VM] Fix incorrect memory allocation

This fixes some hot reload/rollback tests

Change-Id: Ie721977fe7b1de550b854450b2ee6b539f4cacdc
Reviewed-on: https://dart-review.googlesource.com/60961
Reviewed-by: Martin Kustermann <kustermann@google.com>
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 4832667..f31cbb9 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -1025,7 +1025,7 @@
     return;
   }
 
-  *modified_sources = new (zone_) Dart_SourceFile[*count];
+  *modified_sources = zone_->Alloc<Dart_SourceFile>(*count);
   for (intptr_t i = 0; i < *count; ++i) {
     (*modified_sources)[i].uri = modified_sources_uris[i];
     (*modified_sources)[i].source = NULL;