fix truncate call to use extension method

test/codegen/expect/DeltaBlue.js

 dart_library.library('DeltaBlue', null, /* Imports */[
   "dart_runtime/dart",
   'BenchmarkBase',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, BenchmarkBase, core) {
   'use strict';
   let dartx = dart.dartx;
   function main() {
     new DeltaBlue().report();
@@ skipping 268 matching lines @@
         this.offset.removeConstraint(this);
     }
     markInputs(mark) {
       super.markInputs(mark);
       this.scale.mark = this.offset.mark = mark;
     }
     execute() {
       if (this.direction == FORWARD) {
         this.v2.value = dart.notNull(this.v1.value) * dart.notNull(this.scale.value) + dart.notNull(this.offset.value);
       } else {
-        this.v1.value = ((dart.notNull(this.v2.value) - dart.notNull(this.offset.value)) / dart.notNull(this.scale.value)).truncate();
+        this.v1.value = ((dart.notNull(this.v2.value) - dart.notNull(this.offset.value)) / dart.notNull(this.scale.value))[dartx.truncate]();
       }
     }
     recalculate() {
       let ihn = this.input(), out = this.output();
       out.walkStrength = Strength.weakest(this.strength, ihn.walkStrength);
       out.stay = dart.notNull(ihn.stay) && dart.notNull(this.scale.stay) && dart.notNull(this.offset.stay);
       if (dart.notNull(out.stay))
         this.execute();
     }
   }
@@ skipping 259 matching lines @@
   exports.BinaryConstraint = BinaryConstraint;
   exports.ScaleConstraint = ScaleConstraint;
   exports.EqualityConstraint = EqualityConstraint;
   exports.Variable = Variable;
   exports.Planner = Planner;
   exports.Plan = Plan;
   exports.chainTest = chainTest;
   exports.projectionTest = projectionTest;
   exports.change = change;
 });