Remove --experimental-classes flag and related dead code.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 82cfac0fdf3e622a56c4e7ab40f90079b7a85111..db590156a7707cc5d2cfcc772cd46d0c820ca714 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -422,8 +422,7 @@ void FullCodeGenerator::EmitReturnSequence() {
     int no_frame_start = masm_->pc_offset();
 
     int arg_count = info_->scope()->num_parameters() + 1;
-    if (FLAG_experimental_classes &&
-        IsSubclassConstructor(info_->function()->kind())) {
+    if (IsSubclassConstructor(info_->function()->kind())) {
       arg_count++;
     }
     int arguments_bytes = arg_count * kPointerSize;
@@ -3076,15 +3075,7 @@ void FullCodeGenerator::VisitCall(Call* expr) {
       }
     }
   } else if (call_type == Call::SUPER_CALL) {
-    if (FLAG_experimental_classes) {
-      EmitSuperConstructorCall(expr);
-    } else {
-      EmitLoadSuperConstructor();
-      __ Push(result_register());
-      SuperReference* super_ref = callee->AsSuperReference();
-      VisitForStackValue(super_ref->this_var());
-      EmitCall(expr, CallICState::METHOD);
-    }
+    EmitSuperConstructorCall(expr);
   } else {
     DCHECK(call_type == Call::OTHER_CALL);
     // Call to an arbitrary expression not handled specially above.