Fix ToStringVisitor to handle external functions (issue 23968)

pkg/analyzer/lib/src/generated/ast.dart

 // Copyright (c) 2014, 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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.ast;
 
 import 'dart:collection';
@@ skipping 17953 matching lines @@
     _writer.print(";");
     _visitNodeListWithSeparatorAndPrefix(" ", node.updaters, ", ");
     _writer.print(") ");
     _visitNode(node.body);
     return null;
   }
 
   @override
   Object visitFunctionDeclaration(FunctionDeclaration node) {
     _visitNodeListWithSeparatorAndSuffix(node.metadata, " ", " ");
+    _visitTokenWithSuffix(node.externalKeyword, " ");
     _visitNodeWithSuffix(node.returnType, " ");
     _visitTokenWithSuffix(node.propertyKeyword, " ");
     _visitNode(node.name);
     _visitNode(node.functionExpression);
     return null;
   }
 
   @override
   Object visitFunctionDeclarationStatement(FunctionDeclarationStatement node) {
     _visitNode(node.functionDeclaration);
     return null;
   }
 
   @override
   Object visitFunctionExpression(FunctionExpression node) {
     _visitNode(node.typeParameters);
     _visitNode(node.parameters);
-    _writer.print(' ');
+    if (node.body is! EmptyFunctionBody) {
+      _writer.print(' ');
+    }
     _visitNode(node.body);
     return null;
   }
 
   @override
   Object visitFunctionExpressionInvocation(FunctionExpressionInvocation node) {
     _visitNode(node.function);
     _visitNode(node.typeArguments);
     _visitNode(node.argumentList);
     return null;
@@ skipping 2192 matching lines @@
   }
 
   @override
   accept(AstVisitor visitor) => visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _safelyVisitChild(_expression, visitor);
   }
 }