Typecheck constructor initializers properly

test/dart_codegen/expect/convert/latin1.dart

 part of dart.convert;
  const Latin1Codec LATIN1 = const Latin1Codec();
  const int _LATIN1_MASK = 0xFF;
  class Latin1Codec extends Encoding {final bool _allowInvalid;
  const Latin1Codec({
   bool allowInvalid : false}
 ) : _allowInvalid = allowInvalid;
  String get name => "iso-8859-1";
  String decode(List<int> bytes, {
   bool allowInvalid}
@@ skipping 58 matching lines @@
 }
  class _Latin1AllowInvalidDecoderSink extends _Latin1DecoderSink {_Latin1AllowInvalidDecoderSink(StringConversionSink sink) : super(sink);
  void addSlice(List<int> source, int start, int end, bool isLast) {
 RangeError.checkValidRange(start, end, source.length);
  for (int i = start;
  i < end;
  i++) {
 int char = source[i];
  if (char > _LATIN1_MASK || char < 0) {
 if (i > start) _addSliceToSink(source, start, i, false);
- _addSliceToSink(((__x26) => DDC$RT.cast(__x26, DDC$RT.type((List<dynamic> _) {
+ _addSliceToSink(((__x27) => DDC$RT.cast(__x27, DDC$RT.type((List<dynamic> _) {
 }
 ), DDC$RT.type((List<int> _) {
 }
-), "CastLiteral", """line 161, column 25 of dart:convert/latin1.dart: """, __x26 is List<int>, false))(const [0xFFFD]), 0, 1, false);
+), "CastLiteral", """line 161, column 25 of dart:convert/latin1.dart: """, __x27 is List<int>, false))(const [0xFFFD]), 0, 1, false);
  start = i + 1;
 }
 }
  if (start < end) {
 _addSliceToSink(source, start, end, isLast);
 }
  if (isLast) {
 close();
 }
 }
 }