Null check remover understands JS fragments.

pkg/compiler/lib/src/cps_ir/backward_null_check_remover.dart

 library dart2js.cps_ir.backward_null_check_remover;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 import '../common/names.dart';
 import '../universe/selector.dart';
 import 'type_mask_system.dart';
 import 'cps_fragment.dart';
 
 /// Removes null checks that are follwed by another instruction that will
@@ skipping 44 matching lines @@
   Reference<Primitive> getNullCheckedOperand(Primitive prim) {
     if (prim is NullCheck) return prim.value;
     if (prim is GetLength) return prim.object;
     if (prim is GetField) return prim.object;
     if (prim is GetIndex) return prim.object;
     if (prim is SetField) return prim.object;
     if (prim is SetIndex) return prim.object;
     if (prim is InvokeMethod && !selectorsOnNull.contains(prim.selector)) {
       return prim.dartReceiverReference;
     }
+    if (prim is ForeignCode) {
+      return prim.isNullGuardOnNullFirstArgument() ? prim.arguments[0] : null;
+    }
     return null;
   }
 
   /// It has been determined that the null check in [prim] made redundant by
   /// [newNullCheck].  Eliminate [prim] if it is not needed any more.
   void tryEliminateRedundantNullCheck(Primitive prim, Primitive newNullCheck) {
     if (prim is NullCheck) {
       Primitive value = prim.value.definition;
       LetPrim let = prim.parent;
       prim..replaceUsesWith(value)..destroy();
@@ skipping 42 matching lines @@
   void visitLetHandler(LetHandler node) {
     nullCheckedValue = null;
   }
 
   visitInvokeContinuation(InvokeContinuation node) {
     if (!node.isRecursive) {
       nullCheckedValue = nullCheckedValueAt[node.continuation.definition];
     }
   }
 }