fix truncate call to use extension method

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 08b43262ada627d8a6b60442cc7f66f941a0a585..c2917cdf4d5985462eb3f69ebccc1fd36b393214 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -2014,7 +2014,9 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
       // instead of special cases here.
       if (op.type == TokenType.TILDE_SLASH) {
         // `a ~/ b` is equivalent to `(a / b).truncate()`
-        code = '(# / #).truncate()';
+        var div = AstBuilder.binaryExpression(left, '/', right)
+          ..staticType = node.staticType;
+        return _emitSend(div, 'truncate', []);
       } else {
         // TODO(vsm): When do Dart ops not map to JS?
         code = '# $op #';