Typing fixes to eliminate casts/dcalls

lib/runtime/dart/convert.js

 var convert;
 (function(exports) {
   'use strict';
   let ASCII = dart.const(new AsciiCodec());
   let _ASCII_MASK = 127;
   let _allowInvalid = Symbol('_allowInvalid');
   let Codec$ = dart.generic(function(S, T) {
     class Codec extends core.Object {
       Codec() {
       }
@@ skipping 73 matching lines @@
       fuse(other) {
         dart.as(other, Converter$(T, dart.dynamic));
         return new (_FusedConverter$(S, T, dart.dynamic))(this, other);
       }
       startChunkedConversion(sink) {
         dart.as(sink, core.Sink$(T));
         throw new core.UnsupportedError(`This converter does not support chunked conversions: ${this}`);
       }
       bind(source) {
         dart.as(source, async.Stream$(S));
-        return new async.Stream$(T).eventTransformed(source, (sink => new _ConverterStreamEventSink(this, sink)).bind(this));
+        return new (async.Stream$(T)).eventTransformed(source, (sink => new _ConverterStreamEventSink(this, sink)).bind(this));
       }
     }
     Converter[dart.implements] = () => [async.StreamTransformer$(S, T)];
     return Converter;
   });
   let Converter = Converter$();
   class _UnicodeSubsetEncoder extends Converter$(core.String, core.List$(core.int)) {
     _UnicodeSubsetEncoder(subsetMask) {
       this[_subsetMask] = subsetMask;
       super.Converter();
@@ skipping 2318 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.UTF8 = UTF8;
   exports.Utf8Codec = Utf8Codec;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 })(convert || (convert = {}));