Strengthen type assertions post-dominated by checks during type propagation.

runtime/vm/flow_graph_type_propagator.h

Index: runtime/vm/flow_graph_type_propagator.h
diff --git a/runtime/vm/flow_graph_type_propagator.h b/runtime/vm/flow_graph_type_propagator.h
index fafd1b62f26db032a27a5ad8b23f88ae0aa49720..f01dc5f2dd6af2a3e3dcdb9cddf85792749c8602 100644
--- a/runtime/vm/flow_graph_type_propagator.h
+++ b/runtime/vm/flow_graph_type_propagator.h
@@ -38,6 +38,10 @@ class FlowGraphTypePropagator : public FlowGraphVisitor {
   void AddToWorklist(Definition* defn);
   Definition* RemoveLastFromWorklist();
 
+  // Type assertion strengthening.
+  void StrengthenAsserts(BlockEntryInstr* block);
+  void StrengthenAssertWith(Instruction* check);
+
   FlowGraph* flow_graph_;
 
   // Mapping between SSA values and their current reaching types. Valid
@@ -48,6 +52,9 @@ class FlowGraphTypePropagator : public FlowGraphVisitor {
   GrowableArray<Definition*> worklist_;
   BitVector* in_worklist_;
 
+  ZoneGrowableArray<AssertAssignableInstr*>* asserts_;
+  ZoneGrowableArray<intptr_t>* collected_asserts_;
+
   // RollbackEntry is used to track and rollback changed in the types_ array
   // done during dominator tree traversal.
   class RollbackEntry {