anchor.diff:        add(new ir.SetField(environment.lookup(location.box),
                             location.field,
                             initialValue));
  -    } else if (isInMutableVariable(variableElement)) {
  -      add(new ir.LetMutable(getMutableVariable(variableElement),
  -                            initialValue));
       } else {
         initialValue.useElementAsHint(variableElement);
         environment.extend(variableElement, initialValue);
  
positive.code: /// variables in different ways.
  abstract class IrBuilder {
    IrBuilder _makeInstance();
  
    void declareLocalVariable(LocalVariableElement element,
                              {ir.Primitive initialValue});
  
negative.code: 
    final List<ir.Parameter> _parameters = <ir.Parameter>[];
  
    IrBuilderDelimitedState state;
  
    /// A map from variable indexes to their values.
    ///
    /// [BoxLocal]s map to their box. [LocalElement]s that are boxed are not
    /// in the map; look up their [BoxLocal] instead.
    Environment environment;
  
    // The IR builder maintains a context, which is an expression with a hole in
    // it.  The hole represents the focus where new expressions can be added.
  