fix truncate call to use extension method

lib/runtime/dart/_interceptors.js

 dart_library.library('dart/_interceptors', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/_internal',
   'dart/collection',
   'dart/math'
 ], /* Lazy imports */[
   'dart/_js_helper'
 ], function(exports, dart, core, _internal, collection, math, _js_helper) {
   'use strict';
@@ skipping 900 matching lines @@
     [dartx.toUnsigned](width) {
       return this[dartx['&']]((1 << dart.notNull(width)) - 1);
     }
     [dartx.toSigned](width) {
       let signMask = 1 << dart.notNull(width) - 1;
       return dart.notNull(this[dartx['&']](dart.notNull(signMask) - 1)) - dart.notNull(this[dartx['&']](signMask));
     }
     get [dartx.bitLength]() {
       let nonneg = dart.notNull(this[dartx['<']](0)) ? dart.notNull(this[dartx['unary-']]()) - 1 : this;
       if (dart.notNull(nonneg) >= 4294967296) {
-        nonneg = (dart.notNull(nonneg) / 4294967296).truncate();
+        nonneg = (dart.notNull(nonneg) / 4294967296)[dartx.truncate]();
         return dart.notNull(JSInt._bitCount(JSInt._spread(nonneg))) + 32;
       }
       return JSInt._bitCount(JSInt._spread(nonneg));
     }
     static _bitCount(i) {
       i = dart.notNull(JSInt._shru(i, 0)) - (dart.notNull(JSInt._shru(i, 1)) & 1431655765);
       i = (dart.notNull(i) & 858993459) + (dart.notNull(JSInt._shru(i, 2)) & 858993459);
       i = 252645135 & dart.notNull(i) + dart.notNull(JSInt._shru(i, 4));
       i = dart.notNull(i) + dart.notNull(JSInt._shru(i, 8));
       i = dart.notNull(i) + dart.notNull(JSInt._shru(i, 16));
@@ skipping 659 matching lines @@
   exports.JSString = JSString;
   exports.getInterceptor = getInterceptor;
   exports.JSBool = JSBool;
   exports.JSIndexable = JSIndexable;
   exports.JSMutableIndexable = JSMutableIndexable;
   exports.JSObject = JSObject;
   exports.JavaScriptObject = JavaScriptObject;
   exports.PlainJavaScriptObject = PlainJavaScriptObject;
   exports.UnknownJavaScriptObject = UnknownJavaScriptObject;
 });