fix return type comment parsing for top-level generic function

pkg/analyzer/lib/src/generated/parser.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.
 
 library engine.parser;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'ast.dart';
@@ skipping 5171 matching lines @@
             new VariableDeclaration(_createSyntheticIdentifier(), null, null));
         return new TopLevelVariableDeclaration(
             commentAndMetadata.comment,
             commentAndMetadata.metadata,
             new VariableDeclarationList(null, null, keyword, null, variables),
             _expectSemicolon());
       }
       _reportErrorForToken(ParserErrorCode.EXPECTED_EXECUTABLE, _currentToken);
       return null;
     } else if (_tokenMatches(_peek(), TokenType.OPEN_PAREN)) {
+      TypeName returnType = _parseOptionalTypeNameComment();
       _validateModifiersForTopLevelFunction(modifiers);
       return _parseFunctionDeclaration(
-          commentAndMetadata, modifiers.externalKeyword, null);
+          commentAndMetadata, modifiers.externalKeyword, returnType);
     } else if (_peek()
         .matchesAny([TokenType.EQ, TokenType.COMMA, TokenType.SEMICOLON])) {
       if (modifiers.constKeyword == null &&
           modifiers.finalKeyword == null &&
           modifiers.varKeyword == null) {
         _reportErrorForCurrentToken(
             ParserErrorCode.MISSING_CONST_FINAL_VAR_OR_TYPE);
       }
       return new TopLevelVariableDeclaration(
           commentAndMetadata.comment,
@@ skipping 5971 matching lines @@
   }
 
   /**
    * Copy resolution data from the [fromNode] to the [toNode].
    */
   static void copyResolutionData(AstNode fromNode, AstNode toNode) {
     ResolutionCopier copier = new ResolutionCopier();
     copier._isEqualNodes(fromNode, toNode);
   }
 }