Fix for conditional expressions

test/codegen/expect/dart/convert.js

 var convert;
 (function(exports) {
   'use strict';
   let _processed = Symbol('_processed');
   let _original = Symbol('_original');
   // Function _convertJsonToDart: (dynamic, (dynamic, dynamic) → dynamic) → dynamic
   function _convertJsonToDart(json, reviver) {
     dart.assert(reviver !== null);
     // Function walk: (dynamic) → dynamic
     function walk(e) {
@@ skipping 790 matching lines @@
             break;
         }
         if (replace !== null) {
           if (result === null)
             result = new core.StringBuffer(text.substring(start, i));
           result.write(replace);
         } else if (result !== null) {
           result.write(ch);
         }
       }
-      return dart.as(result !== null ? result.toString() : null, core.String);
+      return result !== null ? result.toString() : null;
     }
     startChunkedConversion(sink) {
       if (!dart.is(sink, StringConversionSink)) {
         sink = new StringConversionSink.from(sink);
       }
       return new _HtmlEscapeSink(this, dart.as(sink, StringConversionSink));
     }
   }
   let _escape = Symbol('_escape');
   class _HtmlEscapeSink extends StringConversionSinkBase {
@@ skipping 1499 matching lines @@
   exports.ClosableStringSink = ClosableStringSink;
   exports.StringConversionSinkBase = StringConversionSinkBase;
   exports.StringConversionSinkMixin = StringConversionSinkMixin;
   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 = {}));