fuse some null checks with type checks, introduce a special bool variant

test/codegen/expect/notnull.js

 dart_library.library('notnull', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const notnull = Object.create(null);
   notnull.intAssignments = function() {
     let i = 0;
@@ skipping 137 matching lines @@
     s = s;
     core.print(s + 1);
   };
   dart.fn(notnull.nonNullableCycle, dart.void, []);
   notnull.Foo = class Foo extends core.Object {
     Foo() {
       this.intField = null;
       this.varField = null;
     }
     f(o) {
-      core.print(1 + dart.notNull(dart.as(this.varField, core.num)) + 2);
-      while (dart.notNull(dart.as(dart.dsend(this.varField, '<', 10), core.bool))) {
+      core.print(1 + dart.asNotNull(this.varField, core.num) + 2);

[Jennifer Messerly, 2016/05/10 23:42:12]

`num` and `String` are a couple of other ones I want to take a look at soon.

+      while (dart.test(dart.dsend(this.varField, '<', 10))) {
         this.varField = dart.dsend(this.varField, '+', 1);
       }
-      while (dart.notNull(dart.as(dart.dsend(this.varField, '<', 10), core.bool)))
+      while (dart.test(dart.dsend(this.varField, '<', 10)))
         this.varField = dart.dsend(this.varField, '+', 1);
       core.print(1 + dart.notNull(this.intField) + 2);
       while (dart.notNull(this.intField) < 10) {
         this.intField = dart.notNull(this.intField) + 1;
       }
       while (dart.notNull(this.intField) < 10)
         this.intField = dart.notNull(this.intField) + 1;
       core.print(1 + dart.notNull(o.intField) + 2);
       while (dart.notNull(o.intField) < 10) {
         o.intField = dart.notNull(o.intField) + 1;
@@ skipping 81 matching lines @@
     notnull.localEscapes();
     notnull.controlFlow();
     notnull.cascadesOnNull();
     notnull.nullableCycle();
     notnull.nonNullableCycle();
   };
   dart.fn(notnull.main);
   // Exports:
   exports.notnull = notnull;
 });