Typecheck constructor initializers properly

test/dart_codegen/expect/convert/ascii.dart

 part of dart.convert;
  const AsciiCodec ASCII = const AsciiCodec();
  const int _ASCII_MASK = 0x7F;
  class AsciiCodec extends Encoding {final bool _allowInvalid;
  const AsciiCodec({
   bool allowInvalid : false}
 ) : _allowInvalid = allowInvalid;
  String get name => "us-ascii";
  String decode(List<int> bytes, {
   bool allowInvalid}
@@ skipping 27 matching lines @@
  return DDC$RT.cast(result, DDC$RT.type((List<dynamic> _) {
   }
 ), DDC$RT.type((List<int> _) {
   }
 ), "CastDynamic", """line 96, column 12 of dart:convert/ascii.dart: """, result is List<int>, false);
 }
  StringConversionSink startChunkedConversion(Sink<List<int>> sink) {
 if (sink is! ByteConversionSink) {
   sink = new ByteConversionSink.from(sink);
   }
- return new _UnicodeSubsetEncoderSink(_subsetMask, sink);
+ return new _UnicodeSubsetEncoderSink(_subsetMask, DDC$RT.cast(sink, DDC$RT.type((Sink<List<int>> _) {
+  }
+), ByteConversionSink, "CastGeneral", """line 109, column 55 of dart:convert/ascii.dart: """, sink is ByteConversionSink, true));
 }
  Stream<List<int>> bind(Stream<String> stream) => ((__x0) => DDC$RT.cast(__x0, DDC$RT.type((DDC$async$.Stream<dynamic> _) {
 }
 ), DDC$RT.type((DDC$async$.Stream<List<int>> _) {
 }
 ), "CastDynamic", """line 113, column 52 of dart:convert/ascii.dart: """, __x0 is DDC$async$.Stream<List<int>>, false))(super.bind(stream));
 }
  class AsciiEncoder extends _UnicodeSubsetEncoder {const AsciiEncoder() : super(_ASCII_MASK);
 }
  class _UnicodeSubsetEncoderSink extends StringConversionSinkBase {final ByteConversionSink _sink;
@@ skipping 113 matching lines @@
  RangeError.checkValidRange(start, end, length);
  if (start < end) {
 if (start != 0 || end != length) {
 source = source.sublist(start, end);
 }
  add(source);
 }
  if (isLast) close();
 }
 }