Completion boilerplate for Flutter cons children (flutter-intellij#463)

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

Index: pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
index 0e92c36c30f961861a8980265ccefed327bd7225..f372ae22ac115e1ad6b5037e09c0f20e2ee36c6d 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
@@ -6,12 +6,16 @@ library services.completion.contributor.dart.arglist;
 
 import 'dart:async';
 
+import 'package:analysis_server/src/ide_options.dart';
 import 'package:analysis_server/src/protocol_server.dart'
     hide Element, ElementKind;
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
+import 'package:analysis_server/src/services/completion/dart/utilities.dart';
+import 'package:analysis_server/src/services/correction/flutter_util.dart';
 import 'package:analysis_server/src/utilities/documentation.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 /**
@@ -265,10 +269,51 @@ class ArgListContributor extends DartCompletionContributor {
         _setDocumentation(suggestion, parameter.field?.documentationComment);
         suggestion.element = convertElement(parameter);
       }
+
+      String defaultValue = _getDefaultValue(parameter, request.ideOptions);
+      if (defaultValue != null) {
+        //TODO(pq): unify with `utilities.dart`
+        StringBuffer sb = new StringBuffer();
+        sb.write('${parameter.name}: ');
+        int offset = sb.length;
+        sb.write(defaultValue);
+        suggestion.defaultArgumentListString = defaultValue;
+        suggestion.defaultArgumentListTextRanges = [
+          offset,
+          defaultValue.length
+        ];
+      }
+
       suggestions.add(suggestion);
     }
   }
 
+  //TODO(pq): move and unify w/ `utilities.dart`
+  String _getDefaultValue(ParameterElement param, IdeOptions options) {
+    if (options?.generateFlutterWidgetChildrenBoilerPlate == true) {
+      Element element = param.enclosingElement;
+      if (element is ConstructorElement) {
+        ConstructorElement constructorElement = element;
+        ClassElement classElement = constructorElement.enclosingElement;
+        if (isFlutterWidget(classElement)) {
+          for (ParameterElement param in constructorElement.parameters) {
+            if (param.name == 'children') {
+              DartType type = param.type;
+              if (type is InterfaceType && isDartList(type)) {
+                InterfaceType interfaceType = type;
+                List<DartType> typeArguments = interfaceType.typeArguments;
+                if (typeArguments.length == 1) {
+                  return '<${typeArguments.first.name}>[]';
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+    return null;
+  }
+
   void _addSuggestions(Iterable<ParameterElement> parameters) {
     if (parameters == null || parameters.length == 0) {
       return;