Default arg support continued (flutter-intellij#553).

pkg/analysis_server/lib/src/services/completion/dart/utilities.dart

 // Copyright (c) 2017, 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.
 
 /**
  * A collection of utility methods used by completion contributors.
  */
 import 'package:analysis_server/plugin/protocol/protocol.dart' as protocol
     show Element, ElementKind;
 import 'package:analysis_server/src/protocol_server.dart'
     show CompletionSuggestion, CompletionSuggestionKind, Location;
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
+import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/ast/token.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * The name of the type `dynamic`;
  */
 const DYNAMIC = 'dynamic';
 
 /**
  * A marker used in place of `null` when a function has no return type.
  */
 final TypeName NO_RETURN_TYPE = astFactory.typeName(
     astFactory.simpleIdentifier(new StringToken(TokenType.IDENTIFIER, '', 0)),
     null);
 
 /**
+ * Build a default argument list based on the given [requiredParams] and
+ * [namedParams].
+ */
+String buildDefaultArgList(Iterable<ParameterElement> requiredParams,
+    Iterable<ParameterElement> namedParams) {
+  List<String> args = requiredParams.map((p) => p.name).toList();
+  List<String> requiredArgs = namedParams
+      .where((p) => p.isRequired)
+      .map((p) => '${p.name}: null')
+      .toList();
+  args.addAll(requiredArgs);
+  return args.isEmpty ? null : args.join(', ');
+}
+
+/**
  * Create a new protocol Element for inclusion in a completion suggestion.
  */
 protocol.Element createLocalElement(
     Source source, protocol.ElementKind kind, SimpleIdentifier id,
     {String parameters,
     TypeAnnotation returnType,
     bool isAbstract: false,
     bool isDeprecated: false}) {
   String name;
   Location location;
@@ skipping 106 matching lines @@
     TypeArgumentList typeArgs = type.typeArguments;
     if (typeArgs != null) {
       //TODO (danrubel) include type arguments
     }
     return name;
   } else if (type is GenericFunctionType) {
     // TODO(brianwilkerson) Implement this.
   }
   return DYNAMIC;
 }