Recover correctly from empty optional parameters.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

 // Copyright (c) 2016, 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 fasta.outline_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
@@ skipping 407 matching lines @@
     push(name);
   }
 
   @override
   void endOptionalFormalParameters(
       int count, Token beginToken, Token endToken) {
     debugEvent("OptionalFormalParameters");
     FormalParameterType kind = optional("{", beginToken)
         ? FormalParameterType.NAMED
         : FormalParameterType.POSITIONAL;
-    List parameters = popList(count);
+    // When recovering from an empty list of optional arguments, count may be
+    // 0. It might be simpler if the parser didn't call this method in that
+    // case, however, then [beginOptionalFormalParameters] wouldn't always be
+    // matched by this method.
+    List parameters = popList(count) ?? [];
     for (FormalParameterBuilder parameter in parameters) {
       parameter.kind = kind;
     }
     push(parameters);
   }
 
   @override
   void endFormalParameters(int count, Token beginToken, Token endToken) {
     debugEvent("FormalParameters");
     List formals = popList(count);
@@ skipping 198 matching lines @@
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }