Recover correctly from empty optional parameters.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
 import '../parser/error_kind.dart' show ErrorKind;
 
@@ skipping 1303 matching lines @@
     push(variable);
   }
 
   @override
   void endOptionalFormalParameters(
       int count, Token beginToken, Token endToken) {
     debugEvent("OptionalFormalParameters");
     FormalParameterType kind = optional("{", beginToken)
         ? FormalParameterType.NAMED
         : FormalParameterType.POSITIONAL;
-    push(new OptionalFormals(kind, popList(count)));
+    push(new OptionalFormals(kind, popList(count) ?? []));
   }
 
   @override
   void beginFunctionTypedFormalParameter(Token token) {
     debugEvent("beginFunctionTypedFormalParameter");
     functionNestingLevel++;
   }
 
   @override
   void endFunctionTypedFormalParameter(
@@ skipping 1342 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }