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

test/codegen/expect/notnull.js

Index: test/codegen/expect/notnull.js
diff --git a/test/codegen/expect/notnull.js b/test/codegen/expect/notnull.js
index 2bdaa49f4288ee4579c35c8810b4f2f01abf1891..7e86acf50366deab05ad92f98337e42fdbd66da0 100644
--- a/test/codegen/expect/notnull.js
+++ b/test/codegen/expect/notnull.js
@@ -156,10 +156,10 @@ dart_library.library('notnull', null, /* Imports */[
     }
     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))) {
+      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) {