Add default implementation when fixing missing required closures (flutter-intellij#975)

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/protocol/protocol_generated.dart' as protocol
     show Element, ElementKind;
 import 'package:analysis_server/src/ide_options.dart';
@@ skipping 160 matching lines @@
       String className = classId.name;
       if (className != null && className.length > 0) {
         suggestion.declaringType = className;
       }
     }
   }
   return suggestion;
 }
 
 String getDefaultStringParameterValue(ParameterElement param) {
-  DartType type = param.type;
-  if (type is InterfaceType && isDartList(type)) {
-    List<DartType> typeArguments = type.typeArguments;
-    StringBuffer sb = new StringBuffer();
-    if (typeArguments.length == 1) {
-      DartType typeArg = typeArguments.first;
-      if (!typeArg.isDynamic) {
-        sb.write('<${typeArg.name}>');
+  if (param != null) {
+    DartType type = param.type;
+    if (type is InterfaceType && isDartList(type)) {
+      List<DartType> typeArguments = type.typeArguments;
+      if (typeArguments.length == 1) {
+        DartType typeArg = typeArguments.first;
+        String typeInfo = !typeArg.isDynamic ? '<${typeArg.name}>' : '';
+        return '$typeInfo[]';
       }
-      sb.write('[]');
-      return sb.toString();
     }
+    if (type is FunctionType) {
+      String params = type.parameters.map((p) => p.name).join(', ');
+      //TODO(pq): consider adding a `TODO:` message in generated stub
+      return '($params) {}';
+    }
+    //TODO(pq): support map literals
   }
   return null;
 }
 
 bool isDartList(DartType type) {
   ClassElement element = type.element;
   if (element != null) {
     return element.name == "List" && element.library.isDartCore;
   }
   return false;
@@ skipping 38 matching lines @@
       //TODO (danrubel) include type arguments
     }
     return name;
   } else if (type is GenericFunctionType) {
     // TODO(brianwilkerson) Implement this.
   }
   return DYNAMIC;
 }
 
 String _getDefaultValue(ParameterElement param) => 'null';