anchor.diff:  /// variables in different ways.
   abstract class IrBuilder {
     IrBuilder _makeInstance();
  +
  +  /// True if [local] should currently be accessed from a [ir.MutableVariable].
  +  bool isInMutableVariable(Local local);
  +
  +  /// Creates a [ir.MutableVariable] for the given local.
  +  void makeMutableVariable(Local local);
  +
  +  /// Remove an [ir.MutableVariable] for a local.
  +  ///
  +  /// Subsequent access to the local will be direct rather than through the
  +  /// mutable variable.  This is used for variables that do not spend their
  +  /// entire lifetime as mutable variables (e.g., variables that are boxed
  +  /// in mutable variables for a try block).
  +  void removeMutableVariable(Local local);
   
     void declareLocalVariable(LocalVariableElement element,
                               {ir.Primitive initialValue});
  
positive.code: /// Captured variables are translated to ref cells (see [MutableVariable])
  /// using [GetMutableVariable] and [SetMutableVariable].
  class DartIrBuilder extends IrBuilder {
    final DartIrBuilderSharedState dartState = new DartIrBuilderSharedState();
  
    IrBuilder _makeInstance() => new DartIrBuilder._blank(dartState);
    DartIrBuilder._blank(this.dartState);
  
    DartIrBuilder(ConstantSystem constantSystem,
                  ExecutableElement currentElement,
                  Set<Local> capturedVariables) {
      _init(constantSystem, currentElement);
      capturedVariables.forEach(makeMutableVariable);
    }
  
    @override
    bool isInMutableVariable(Local local) {
      return mutableVariables.containsKey(local) &&
             !dartState.registerizedMutableVariables.contains(local);
    }
  
    void _enterScope(ClosureScope scope) {
  
negative.code:       add(new ir.SetField(environment.lookup(location.box),
                            location.field,
                            initialValue));
      } else {
        initialValue.useElementAsHint(variableElement);
        environment.extend(variableElement, initialValue);
  