Allow trailing comma in assert

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index 158d093368ca1b8225c2a49359dbd30a98229af3..2b383646f3415a546bd498af3c6d343bbb039383 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -801,10 +801,19 @@ class Parser {
     Expression message;
     if (_matches(TokenType.COMMA)) {
       comma = getAndAdvance();
-      message = parseExpression2();
+      if (_matches(TokenType.CLOSE_PAREN)) {
+        comma = null;
+      } else {
+        message = parseExpression2();
+        if (_matches(TokenType.COMMA)) {
+          getAndAdvance();
+        }
+      }
     }
     Token rightParen = _expect(TokenType.CLOSE_PAREN);
     Token semicolon = _expect(TokenType.SEMICOLON);
+    // TODO(brianwilkerson) We should capture the trailing comma in the AST, but
+    // that would be a breaking change, so we drop it for now.
     return astFactory.assertStatement(
         keyword, leftParen, expression, comma, message, rightParen, semicolon);
   }