Make int and double nullable by default

test/codegen/expect/fieldtest.js

Index: test/codegen/expect/fieldtest.js
diff --git a/test/codegen/expect/fieldtest.js b/test/codegen/expect/fieldtest.js
index d19e22834e28bfd5d7ae366231a9e39c0fea834f..f2cc022fea94f8795f579b743d7f019f23d133b3 100644
--- a/test/codegen/expect/fieldtest.js
+++ b/test/codegen/expect/fieldtest.js
@@ -9,9 +9,9 @@ var fieldtest;
   let B$ = dart.generic(function(T) {
     class B extends dart.Object {
       B() {
-        this.x = dart.as(null, core.int);
+        this.x = null;
         this.y = null;
-        this.z = dart.as(null, T);
+        this.z = null;
       }
     }
     return B;
@@ -37,7 +37,7 @@ var fieldtest;
   }
   dart.defineLazyProperties(exports, {
     get y() {
-      return compute() + 444;
+      return dart.notNull(compute()) + 444;
     },
     set y(_) {}
   });