anchor.diff:    }
   
     void forgetElement(Element element) {
  -    _initialVariableValues.remove(element);
  +    initialVariableValues.remove(element);
       if (element is ScopeContainerElement) {
  -      element.forEachLocalMember(_initialVariableValues.remove);
  +      element.forEachLocalMember(initialVariableValues.remove);
       }
       if (element is FunctionElement && element.hasFunctionSignature) {
         element.functionSignature.forEachParameter(this.forgetElement);
  
positive.code:     }
      if (expression != null) {
        element.constant = expression;
        _initialVariableValues[element.declaration] = expression;
      } else {
        assert(invariant(element, !isConst,
            message: "Variable $element does not compile to a constant."));
  
negative.code:    *
     * Invariant: The keys in this map are declarations.
     */
    // TODO(johnniwinther): Make this purely internal when no longer used by
    // poi/forget_element_test.
    final Map<VariableElement, ConstantExpression> initialVariableValues =
        new Map<VariableElement, ConstantExpression>();
  
    /** The set of variable elements that are in the process of being computed. */
  