fix truncate call to use extension method

lib/runtime/dart/_internal.js

 dart_library.library('dart/_internal', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/math',
   'dart/_interceptors',
   'dart/_js_primitives'
 ], function(exports, dart, core, collection, math, _interceptors, _js_primitives) {
   'use strict';
@@ skipping 2056 matching lines @@
         let j = i;
         while (dart.notNull(j) > dart.notNull(left) && dart.notNull(dart.dcall(compare, a[dartx.get](dart.notNull(j) - 1), el)) > 0) {
           a[dartx.set](j, a[dartx.get](dart.notNull(j) - 1));
           j = dart.notNull(j) - 1;
         }
         a[dartx.set](j, el);
       }
     }
     static _dualPivotQuicksort(a, left, right, compare) {
       dart.assert(dart.notNull(right) - dart.notNull(left) > dart.notNull(Sort._INSERTION_SORT_THRESHOLD));
-      let sixth = ((dart.notNull(right) - dart.notNull(left) + 1) / 6).truncate();
+      let sixth = ((dart.notNull(right) - dart.notNull(left) + 1) / 6)[dartx.truncate]();
       let index1 = dart.notNull(left) + dart.notNull(sixth);
       let index5 = dart.notNull(right) - dart.notNull(sixth);
-      let index3 = ((dart.notNull(left) + dart.notNull(right)) / 2).truncate();
+      let index3 = ((dart.notNull(left) + dart.notNull(right)) / 2)[dartx.truncate]();
       let index2 = dart.notNull(index3) - dart.notNull(sixth);
       let index4 = dart.notNull(index3) + dart.notNull(sixth);
       let el1 = a[dartx.get](index1);
       let el2 = a[dartx.get](index2);
       let el3 = a[dartx.get](index3);
       let el4 = a[dartx.get](index4);
       let el5 = a[dartx.get](index5);
       if (dart.notNull(dart.dcall(compare, el1, el2)) > 0) {
         let t = el1;
         el1 = el2;
@@ skipping 355 matching lines @@
   exports.ReversedListIterable = ReversedListIterable;
   exports.UnmodifiableListError = UnmodifiableListError;
   exports.NonGrowableListError = NonGrowableListError;
   exports.makeListFixedLength = makeListFixedLength;
   exports.Lists = Lists;
   exports.printToConsole = printToConsole;
   exports.Sort = Sort;
   exports.Symbol = Symbol;
   exports.POWERS_OF_TEN = POWERS_OF_TEN;
 });