fixes #40, extension methods for primitive types

test/codegen/expect/BenchmarkBase.js

 var BenchmarkBase = dart.defineLibrary(BenchmarkBase, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
         throw `Values not equal: ${expected} vs ${actual}`;
       }
     }
     static listEquals(expected, actual) {
-      if (expected[core.$length] != actual[core.$length]) {
-        throw `Lists have different lengths: ${expected[core.$length]} vs ${actual[core.$length]}`;
+      if (expected.length != actual.length) {
+        throw `Lists have different lengths: ${expected.length} vs ${actual.length}`;
       }
-      for (let i = 0; dart.notNull(i) < dart.notNull(actual[core.$length]); i = dart.notNull(i) + 1) {
-        Expect.equals(expected[core.$get](i), actual[core.$get](i));
+      for (let i = 0; dart.notNull(i) < dart.notNull(actual.length); i = dart.notNull(i) + 1) {
+        Expect.equals(expected[dartx.get](i), actual[dartx.get](i));
       }
     }
     fail(message) {
       throw message;
     }
   }
   dart.setSignature(Expect, {
     methods: () => ({fail: [core.Object, [core.Object]]}),
     statics: () => ({
       equals: [dart.void, [core.Object, core.Object]],
@@ skipping 56 matching lines @@
       measure: [core.double, []],
       report: [dart.void, []]
     }),
     statics: () => ({measureFor: [core.double, [core.Function, core.int]]}),
     names: ['measureFor']
   });
   // Exports:
   exports.Expect = Expect;
   exports.BenchmarkBase = BenchmarkBase;
 })(BenchmarkBase, core);