anchor.diff:  
     @override
     TypeExpr visitAsExpression(AsExpression node) {
  -    TypeExpr operand = _visit(node.operand);
  -    Type type = _typesBuilder.fromStaticType(node.type, true);
  -    TypeExpr runtimeType = _translator.translate(node.type);
  -    TypeExpr result = new TypeCheck(operand, runtimeType, node, type);
  +    final TypeExpr operand = _visit(node.operand);
  +    final TypeExpr result = _typeCheck(operand, node.type, node);
       explicitCasts[node] = result;
  -    _summary.add(result);
       return result;
     }
   
  
positive.code: 
    void _handleNestedFunctionNode(FunctionNode node) {
      final oldReturn = _returnValue;
      final oldVariableJoins = _variableJoins;
      final oldVariables = _variables;
      _returnValue = null;
      _variableJoins = <VariableDeclaration, Join>{};
      _variableJoins.addAll(oldVariableJoins);
      _variables = <VariableDeclaration, TypeExpr>{};
      _variables.addAll(oldVariables);
  
      // Approximate parameters of nested functions with static types.
  
negative.code:       int count = firstParamIndex;
        for (int i = 0; i < function.positionalParameters.length; ++i) {
          final decl = function.positionalParameters[i];
          final type = useTypesFrom.positionalParameters[i].type;
          TypeExpr param = _summary.statements[count++];
          if (_useTypeCheckForParameter(decl)) {
            param = _typeCheck(param, type, decl);
          }
          final Join v = _declareVariable(decl, type: type);
          v.values.add(param);
        }
        for (int i = 0; i < function.namedParameters.length; ++i) {
          final decl = function.namedParameters[i];
          final type = useTypesFrom.namedParameters[i].type;
          TypeExpr param = _summary.statements[count++];
          if (_useTypeCheckForParameter(decl)) {
            param = _typeCheck(param, type, decl);
          }
          final Join v = _declareVariable(decl, type: type);
          v.values.add(param);
        }
        assertx(count == _summary.parameterCount);
  
  