Typecheck constructor initializers properly

test/dart_codegen/expect/convert/line_splitter.dart

 part of dart.convert;
  class LineSplitter extends Converter<String, List<String>> {const LineSplitter();
  List<String> convert(String data) {
   var lines = new List<String>();
    _LineSplitterSink._addSlice(data, 0, data.length, true, lines.add);
    return lines;
   }
  StringConversionSink startChunkedConversion(Sink<String> sink) {
   if (sink is! StringConversionSink) {
     sink = new StringConversionSink.from(sink);
     }
-   return new _LineSplitterSink(sink);
+   return new _LineSplitterSink(DDC$RT.cast(sink, DDC$RT.type((Sink<String> _) {
+    }
+  ), StringConversionSink, "CastGeneral", """line 26, column 34 of dart:convert/line_splitter.dart: """, sink is StringConversionSink, true));
   }
 }
  class _LineSplitterSink extends StringConversionSinkBase {static const int _LF = 10;
  static const int _CR = 13;
  final StringConversionSink _sink;
  String _carry;
  _LineSplitterSink(this._sink);
  void addSlice(String chunk, int start, int end, bool isLast) {
 if (_carry != null) {
   chunk = _carry + chunk.substring(start, end);
@@ skipping 39 matching lines @@
    if (isLast) {
     adder(carry);
     }
    else {
     return carry;
     }
   }
  return null;
 }
 }