Remove checked mode instrinsic for _List.[]=.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index e48901261802913e89660834110017d20bb12967..4598ddd37485a4e06dae9635863ceef0d5f3ee17 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -37,10 +37,11 @@ void Intrinsifier::ObjectArraySetIndexed(Assembler* assembler) {
   if (Isolate::Current()->flags().type_checks()) {
     return;
   }
+
+  Label fall_through;
   __ movq(RDX, Address(RSP, + 1 * kWordSize));  // Value.
   __ movq(RCX, Address(RSP, + 2 * kWordSize));  // Index.
   __ movq(RAX, Address(RSP, + 3 * kWordSize));  // Array.
-  Label fall_through;
   __ testq(RCX, Immediate(kSmiTagMask));
   __ j(NOT_ZERO, &fall_through);
   // Range check.