Optional trailing comma support for list and map literals.

pkg/analyzer_experimental/lib/src/services/formatter_impl.dart

 // Copyright (c) 2013, 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.
 
 library formatter_impl;
 
 import 'dart:math';
 
 import 'package:analyzer_experimental/analyzer.dart';
 import 'package:analyzer_experimental/src/generated/parser.dart';
@@ skipping 781 matching lines @@
 
   visitLibraryIdentifier(LibraryIdentifier node) {
     append(node.name);
   }
 
   visitListLiteral(ListLiteral node) {
     modifier(node.constKeyword);
     visit(node.typeArguments);
     token(node.leftBracket);
     visitNodes(node.elements, separatedBy: commaSeperator);
+    optionalTrailingComma(node.rightBracket);
     token(node.rightBracket);
   }
 
   visitMapLiteral(MapLiteral node) {
     modifier(node.constKeyword);
     visitNode(node.typeArguments, followedBy: space);
     token(node.leftBracket);
     visitNodes(node.entries, separatedBy: commaSeperator);
+    optionalTrailingComma(node.rightBracket);
     token(node.rightBracket);
   }
-
+  
   visitMapLiteralEntry(MapLiteralEntry node) {
     visit(node.key);
     token(node.separator);
     space();
     visit(node.value);
   }
 
   visitMethodDeclaration(MethodDeclaration node) {
     modifier(node.externalKeyword);
     modifier(node.modifierKeyword);
@@ skipping 323 matching lines @@
     }
   }
 
 
   /// Emit the given [modifier] if it's non null, followed by non-breaking
   /// whitespace.
   modifier(Token modifier) {
     token(modifier, followedBy: space);
   }
 
-
   /// Indicate that at least one newline should be emitted and possibly more
   /// if the source has them.
   newlines() {
     needsNewline = true;
   }
 
+  /// Optionally emit a trailing comma.
+  optionalTrailingComma(Token rightBracket) {
+    if (rightBracket.previous.lexeme == ',') {
+      comma();
+    }
+  }
+  
   token(Token token, {precededBy(), followedBy(), int minNewlines: 0}) {
     if (token != null) {
       if (needsNewline) {
         minNewlines = max(1, minNewlines);
       }
       var emitted = emitPrecedingCommentsAndNewlines(token, min: minNewlines);
       if (emitted > 0) {
         needsNewline = false;
       }
-      if (precededBy !=null) {
+      if (precededBy != null) {
         precededBy();
       }
       append(token.lexeme);
       if (followedBy != null) {
         followedBy();
       }
       previousToken = token;
     }
   }
 
@@ skipping 157 matching lines @@
     var lastLine =
         lineInfo.getLocation(lastOffset).lineNumber;
     var currentLine =
         lineInfo.getLocation(currentOffset).lineNumber;
     return currentLine - lastLine;
   }
 
   String toString() => writer.toString();
 
 }