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

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 3cd869b03f61ae03a8232c95f5ca021e020124d3..48b4dd14f10a9df12d1809b3454f99b6dcc2f572 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -4091,13 +4091,20 @@ class FunctionType extends DartType {
   final int requiredParameterCount;
   final List<DartType> positionalParameters;
   final List<NamedType> namedParameters; // Must be sorted.
+
+  /// The optional names of [positionalParameters], might be empty, but
+  /// not `null`.

[Siggi Cherem (dart-lang), 2017/07/31 17:43:08]

it might be a bit confusing if positionalParameters.length and
positionalParameterNames.length have different length (even if 0), is there a
reason why we can't use `null` as a default instead?

[scheglov, 2017/07/31 20:35:30]

In flatbuffers every type has default value. For lists default value is an empty
list. This makes life easier - we don't need separate flags to mark null values
- always just length and values.

+  @informative

[Siggi Cherem (dart-lang), 2017/07/31 17:43:07]

General question about what we should do about @informative fields: I know we
want to support having .dill files with and without this data (so we can exclude
it when it is not needed). What are your thoughts on how to exclude it:
 - compute it in fasta always, then omit it during serialization
 - make fasta smarter to optionally produce the data (e.g. use a compiler-option
to ensure they are built), then serialize in the same way regardless.

[scheglov, 2017/07/31 20:35:30]

Well, ultimately any computation, which results we don't use, is useless and
should be prevented.
So, I think we should stop computing these values where it is cheaper to skip
than to compute.

Nothing prevents us from doing this at any point in time.

+  final List<String> positionalParameterNames;
+
   final DartType returnType;
   int _hashCode;
 
   FunctionType(List<DartType> positionalParameters, this.returnType,
       {this.namedParameters: const <NamedType>[],
       this.typeParameters: const <TypeParameter>[],
-      int requiredParameterCount})
+      int requiredParameterCount,
+      this.positionalParameterNames: const <String>[]})
       : this.positionalParameters = positionalParameters,
         this.requiredParameterCount =
             requiredParameterCount ?? positionalParameters.length;