fixes #197, don't generate casts that always pass

test/codegen/expect/unittest.js

 dart_library.library('unittest', null, /* Imports */[
   "dart_runtime/dart",
   'dom/dom',
   'dart/core',
   'dart/async'
 ], /* Lazy imports */[
 ], function(exports, dart, dom, core, async) {
   'use strict';
   let dartx = dart.dartx;
   function group(name, body) {
-    return dart.dsend(dart.as(dom.window, dart.dynamic), 'suite', name, body);
+    return dart.dsend(dom.window, 'suite', name, body);
   }
   dart.fn(group, dart.void, [core.String, dart.functionType(dart.void, [])]);
   function test(name, body, opts) {
     let skip = opts && 'skip' in opts ? opts.skip : null;
     if (skip != null) {
       core.print(`SKIP ${name}: ${skip}`);
       return;
     }
-    dart.dsend(dart.as(dom.window, dart.dynamic), 'test', name, dart.fn(done => {
+    dart.dsend(dom.window, 'test', name, dart.fn(done => {
       function _finishTest(f) {
         if (dart.is(f, async.Future)) {
           dart.dsend(f, 'then', _finishTest);
         } else {
           dart.dcall(done);
         }
       }
       dart.fn(_finishTest);
       _finishTest(body());
     }));
@@ skipping 97 matching lines @@
   exports.isNot = isNot;
   exports.isTrue = isTrue;
   exports.isNull = isNull;
   exports.isRangeError = isRangeError;
   exports.isNoSuchMethodError = isNoSuchMethodError;
   exports.lessThan = lessThan;
   exports.greaterThan = greaterThan;
   exports.throwsA = throwsA;
   exports.Matcher = Matcher;
 });