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

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index c9a9df7d42c645b409f37a7155f5856915f6c858..bee10e5575be81cda90095067cc9734f9cbf097f 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1111,7 +1111,7 @@ Definition* AssertAssignableInstr::Canonicalize(FlowGraphOptimizer* optimizer) {
     return value()->definition();
   }
 
-  // (3) For uninstantiated target types: If the instantiator type arguments
+  // For uninstantiated target types: If the instantiator type arguments
   // are constant, instantiate the target type here.
   if (dst_type().IsInstantiated()) return this;