Recover correctly from empty optional parameters.

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

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index 2f9304c0b798e1312876328322e154cd43b8bf54..215d2eb04fc582f20332322e26b1b2ac5526bb38 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -425,7 +425,11 @@ class OutlineBuilder extends UnhandledListener {
     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;
     }