Remove signature classes from the VM.

runtime/vm/flow_graph_inliner.cc

Index: runtime/vm/flow_graph_inliner.cc
diff --git a/runtime/vm/flow_graph_inliner.cc b/runtime/vm/flow_graph_inliner.cc
index 32be459f860f5989d7f0fe4d503de90abba4e78e..1b39f976e7d126cab98f2a7aa253a06e6771657c 100644
--- a/runtime/vm/flow_graph_inliner.cc
+++ b/runtime/vm/flow_graph_inliner.cc
@@ -1128,14 +1128,13 @@ class CallSiteInliner : public ValueObject {
           call->ArgumentAt(0)->OriginalDefinition()->AsAllocateObject();
       if ((alloc != NULL) && !alloc->closure_function().IsNull()) {
         target ^= alloc->closure_function().raw();
-        ASSERT(target.signature_class() == alloc->cls().raw());
+        ASSERT(alloc->cls().IsClosureClass());
       }
       ConstantInstr* constant =
           call->ArgumentAt(0)->OriginalDefinition()->AsConstant();
       if ((constant != NULL) &&
-          constant->value().IsInstance() &&
-          Instance::Cast(constant->value()).IsClosure()) {
-        target ^= Closure::function(Instance::Cast(constant->value()));
+          constant->value().IsClosure()) {
+        target ^= Closure::Cast(constant->value()).function();
       }
 
       if (target.IsNull()) {