Do not allow creation of signature classes in the background compilation path until we have the fix…

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index f10e60ca70622f8a7ab08ec966de4db0076ca3de..c36bf537aceae275156d3662cef6e07f1461f23c 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -225,6 +225,14 @@ bool FlowGraphOptimizer::TryCreateICData(InstanceCallInstr* call) {
       // finalized yet.
       return false;
     }
+    // Do not run the optimization below if in background compilation since
+    // resolution of method extractor functions may create new signature
+    // classes.
+    // TODO(regis): Remove test for background compilation once signature
+    // classes are not generated any longer.
+    if (!thread()->IsMutatorThread()) {
+      return false;
+    }
     const Array& args_desc_array = Array::Handle(Z,
         ArgumentsDescriptor::New(call->ArgumentCount(),
                                  call->argument_names()));