fix truncate call to use extension method

test/codegen/expect/expect.js

 dart_library.library('expect', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class Expect extends core.Object {
     static _truncateString(string, start, end, length) {
       if (dart.notNull(end) - dart.notNull(start) > dart.notNull(length)) {
         end = dart.notNull(start) + dart.notNull(length);
       } else if (dart.notNull(end) - dart.notNull(start) < dart.notNull(length)) {
         let overflow = dart.notNull(length) - (dart.notNull(end) - dart.notNull(start));
         if (dart.notNull(overflow) > 10)
           overflow = 10;
-        start = dart.notNull(start) - ((dart.notNull(overflow) + 1) / 2).truncate();
-        end = dart.notNull(end) + (dart.notNull(overflow) / 2).truncate();
+        start = dart.notNull(start) - ((dart.notNull(overflow) + 1) / 2)[dartx.truncate]();
+        end = dart.notNull(end) + (dart.notNull(overflow) / 2)[dartx.truncate]();
         if (dart.notNull(start) < 0)
           start = 0;
         if (dart.notNull(end) > dart.notNull(string[dartx.length]))
           end = string[dartx.length];
       }
       if (start == 0 && end == string[dartx.length])
         return string;
       let buf = new core.StringBuffer();
       if (dart.notNull(start) > 0)
         buf.write("...");
       for (let i = start; dart.notNull(i) < dart.notNull(end); i = dart.notNull(i) + 1) {
         let code = string[dartx.codeUnitAt](i);
         if (dart.notNull(code) < 32) {
           buf.write("\\x");
-          buf.write("0123456789abcdef"[dartx.get]((dart.notNull(code) / 16).truncate()));
+          buf.write("0123456789abcdef"[dartx.get]((dart.notNull(code) / 16)[dartx.truncate]()));
           buf.write("0123456789abcdef"[dartx.get](dart.notNull(code) % 16));
         } else {
           buf.writeCharCode(string[dartx.codeUnitAt](i));
         }
       }
       if (dart.notNull(end) < dart.notNull(string[dartx.length]))
         buf.write("...");
       return dart.toString(buf);
     }
     static _stringDifference(expected, actual) {
@@ skipping 289 matching lines @@
   dart.setSignature(AssumeDynamic, {
     constructors: () => ({AssumeDynamic: [AssumeDynamic, []]})
   });
   // Exports:
   exports.Expect = Expect;
   exports.ExpectException = ExpectException;
   exports.NoInline = NoInline;
   exports.TrustTypeAnnotations = TrustTypeAnnotations;
   exports.AssumeDynamic = AssumeDynamic;
 });