In checked mode, parameters are already explicitly checked in the inferrer graph. So there is no ne…

sdk/lib/_internal/compiler/implementation/inferrer/type_graph_nodes.dart

Index: sdk/lib/_internal/compiler/implementation/inferrer/type_graph_nodes.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/inferrer/type_graph_nodes.dart	(revision 29532)
+++ sdk/lib/_internal/compiler/implementation/inferrer/type_graph_nodes.dart	(working copy)
@@ -304,6 +304,8 @@
   TypeMask potentiallyNarrowType(TypeMask mask,
                                  TypeGraphInferrerEngine inferrer) {
     Compiler compiler = inferrer.compiler;
+    // Parameters are being explicitly checked in the method.
+    if (element.isParameter() || element.isFieldParameter()) return mask;
     if (!compiler.trustTypeAnnotations && !compiler.enableTypeAssertions) {
       return mask;
     }