Typecheck constructor initializers properly

test/dart_codegen/expect/convert/html_escape.dart

 part of dart.convert;
  const HtmlEscape HTML_ESCAPE = const HtmlEscape();
  class HtmlEscapeMode {final String _name;
  final bool escapeLtGt;
  final bool escapeQuot;
  final bool escapeApos;
  final bool escapeSlash;
  static const HtmlEscapeMode UNKNOWN = const HtmlEscapeMode._('unknown', true, true, true, true);
  static const HtmlEscapeMode ATTRIBUTE = const HtmlEscapeMode._('attribute', false, true, false, false);
  static const HtmlEscapeMode ELEMENT = const HtmlEscapeMode._('element', true, false, false, true);
@@ skipping 33 matching lines @@
  else if (result != null) {
   result.write(ch);
   }
 }
  return ((__x6) => DDC$RT.cast(__x6, dynamic, String, "CastGeneral", """line 72, column 12 of dart:convert/html_escape.dart: """, __x6 is String, true))(result != null ? result.toString() : null);
 }
  StringConversionSink startChunkedConversion(Sink<String> sink) {
 if (sink is! StringConversionSink) {
 sink = new StringConversionSink.from(sink);
 }
- return new _HtmlEscapeSink(this, sink);
+ return new _HtmlEscapeSink(this, DDC$RT.cast(sink, DDC$RT.type((Sink<String> _) {
+}
+), StringConversionSink, "CastGeneral", """line 79, column 38 of dart:convert/html_escape.dart: """, sink is StringConversionSink, true));
 }
 }
  class _HtmlEscapeSink extends StringConversionSinkBase {final HtmlEscape _escape;
  final StringConversionSink _sink;
  _HtmlEscapeSink(this._escape, this._sink);
  void addSlice(String chunk, int start, int end, bool isLast) {
 var val = _escape._convert(chunk, start, end);
  if (val == null) {
 _sink.addSlice(chunk, start, end, isLast);
 }
  else {
 _sink.add(val);
  if (isLast) _sink.close();
 }
 }
  void close() => _sink.close();
 }