anchor.diff:  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;
  
    IrBuilder _makeInstance() => new DartIrBuilder._blank(dartState);
    DartIrBuilder._blank(this.dartState);
  
    DartIrBuilder(ConstantSystem constantSystem,
                  ExecutableElement currentElement,
                  Set<Local> capturedVariables)
        : dartState = new DartIrBuilderSharedState(capturedVariables) {
      _init(constantSystem, currentElement);
    }
  
    bool isInMutableVariable(Local local) {
      return dartState.local2mutable.containsKey(local) &&
             !dartState.registerizedMutableVariables.contains(local);
    }
  
    void makeMutableVariable(Local local) {
      dartState.makeMutableVariable(local);
    }
  
    void removeMutableVariable(Local local) {
      dartState.local2mutable.remove(local);
    }
  
    /// Gets the [MutableVariable] containing the value of [local].
    ir.MutableVariable getMutableVariable(Local local) {
      return dartState.local2mutable[local];
    }
  
    void _enterScope(ClosureScope scope) {
  
negative.code: 
    final List<ir.Definition> functionParameters = <ir.Definition>[];
  
    IrBuilderSharedState(this.constantSystem, this.currentElement);
  
    ir.Parameter get thisParameter => _thisParameter;
    void set thisParameter(ir.Parameter value) {
  