anchor.diff:  
     int arrowFunctionOut(ArrowFunction fun, VarCollector vars) {
       // TODO: support static, get/set, async, and generators.
  -    if (fun.params.length == 1 && fun.params.first is VariableReference) {
  +    if (fun.params.length == 1) {
         visitNestedExpression(fun.params.single, ASSIGNMENT,
             newInForInit: false, newAtStatementBegin: false);
       } else {
         out("(");
  -      if (fun.params != null) {
  -        visitCommaSeparated(fun.params, PRIMARY,
  -            newInForInit: false, newAtStatementBegin: false);
  -      }
  +      visitCommaSeparated(fun.params, PRIMARY,
  +          newInForInit: false, newAtStatementBegin: false);
  +
         out(")");
       }
       out("=>");
  
positive.code: // Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
  // for details. All rights reserved. Use of this source code is governed by a
  // BSD-style license that can be found in the LICENSE file.
  
  // @dart = 2.10
  
  library js.size_estimator;
  
  
negative.code:     out('return'); // 'return'
      if (node.value != null) {
        pendingSpace = true;
        visitNestedExpression(node.value!, EXPRESSION,
            newInForInit: false, newAtStatementBegin: false);
      }
      outSemicolonLn();
  