Add optional FunctionType.positionalParameterNames and use them to resynthesize function-type forma…

pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
index 437379a81070f3e67ff5e84fc436693903e33f56..3dc642ead97073bfafae5385c1f717a8043adb8e 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
@@ -41,6 +41,7 @@ class KernelFunctionTypeBuilder extends FunctionTypeBuilder
     DartType builtReturnType =
         returnType?.build(library) ?? const DynamicType();
     List<DartType> positionalParameters = <DartType>[];
+    List<String> positionalParameterNames = <String>[];
     List<NamedType> namedParameters;
     int requiredParameterCount = 0;
     if (formals != null) {
@@ -48,6 +49,7 @@ class KernelFunctionTypeBuilder extends FunctionTypeBuilder
         DartType type = formal.type?.build(library) ?? const DynamicType();
         if (formal.isPositional) {
           positionalParameters.add(type);
+          positionalParameterNames.add(formal.name ?? '');
           if (formal.isRequired) requiredParameterCount++;
         } else if (formal.isNamed) {
           namedParameters ??= <NamedType>[];
@@ -68,7 +70,8 @@ class KernelFunctionTypeBuilder extends FunctionTypeBuilder
     return new FunctionType(positionalParameters, builtReturnType,
         namedParameters: namedParameters ?? const <NamedType>[],
         typeParameters: typeParameters ?? const <TypeParameter>[],
-        requiredParameterCount: requiredParameterCount);
+        requiredParameterCount: requiredParameterCount,
+        positionalParameterNames: positionalParameterNames);
   }
 
   Supertype buildSupertype(LibraryBuilder library) {