Typecheck constructor initializers properly

test/codegen/expect/convert/convert.js

 var convert;
 (function(convert) {
   'use strict';
   // Function _convertJsonToDart: (dynamic, (dynamic, dynamic) → dynamic) → dynamic
   function _convertJsonToDart(json, reviver) {
     dart.assert(reviver !== null);
     // Function walk: (dynamic) → dynamic
     function walk(e) {
       if (dart.dbinary(dart.dinvokef(/* Unimplemented unknown name */JS, 'bool', '# == null', e), '||', dart.dinvokef(/* Unimplemented unknown name */JS, 'bool', 'typeof # != "object"', e))) {
         return e;
@@ skipping 1042 matching lines @@
       return _convertJsonToDart(parsed, reviver);
     }
   }
   // Function _defaultToEncodable: (dynamic) → Object
   function _defaultToEncodable(object) {
     return dart.dinvoke(object, 'toJson');
   }
   class _JsonStringifier extends dart.Object {
     _JsonStringifier(_toEncodable) {
       this._seen = new core.List();
-      this._toEncodable = _toEncodable !== null ? _toEncodable : _defaultToEncodable;
+      this._toEncodable = dart.as(_toEncodable !== null ? _toEncodable : _defaultToEncodable, core.Function);
     }
     static hexDigit(x) {
       return x < 10 ? 48 + x : 87 + x;
     }
     writeStringContent(s) {
       let offset = 0;
       let length = s.length;
       for (let i = 0; i < length; i++) {
         let charCode = s.codeUnitAt(i);
         if (charCode > BACKSLASH)
@@ skipping 1187 matching lines @@
   convert.ClosableStringSink = ClosableStringSink;
   convert.StringConversionSinkBase = StringConversionSinkBase;
   convert.StringConversionSinkMixin = StringConversionSinkMixin;
   convert.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   convert.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   convert.UTF8 = UTF8;
   convert.Utf8Codec = Utf8Codec;
   convert.Utf8Encoder = Utf8Encoder;
   convert.Utf8Decoder = Utf8Decoder;
 })(convert || (convert = {}));