anchor.diff:  
     void visitInvokeMethod(InvokeMethod node) {
       AbstractValue receiver = getValue(node.receiver.definition);
  +    node.receiverIsNotNull = receiver.isDefinitelyNotNull;
       if (receiver.isNothing) {
         return;  // And come back later.
       }
  
positive.code:   @override
    void visitGetLength(GetLength node) {
      AbstractValue input = getValue(node.object.definition);
      int length = typeSystem.getContainerLength(input.type);
      if (length != null) {
        // TODO(asgerf): Constant-folding the length might degrade the VM's
  
negative.code:         CpsFragment cps = new CpsFragment(sourceInfo);
          cps.invokeBuiltin(BuiltinMethod.Push,
              list,
              <Primitive>[addedItem]);
          cps.invokeContinuation(cont, [cps.makeNull()]);
          replaceSubtree(node, cps.result);
          push(cps.result);
  