Make int and double nullable by default

test/codegen/expect/fieldtest.js

 var fieldtest;
 (function(exports) {
   'use strict';
   class A extends dart.Object {
     A() {
       this.x = 42;
     }
   }
   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;
   });
   let B = B$(dynamic);
   // Function foo: (A) → int
   function foo(a) {
     core.print(a.x);
     return a.x;
   }
   // Function bar: (dynamic) → int
   function bar(a) {
     core.print(dart.dload(a, 'x'));
     return dart.as(dart.dload(a, 'x'), core.int);
   }
   // Function baz: (A) → dynamic
   function baz(a) {
     return a.x;
   }
   // Function compute: () → int
   function compute() {
     return 123;
   }
   dart.defineLazyProperties(exports, {
     get y() {
-      return compute() + 444;
+      return dart.notNull(compute()) + 444;
     },
     set y(_) {}
   });
   dart.copyProperties(exports, {
     get q() {
       return core.String['+'](core.String['+']('life, ', 'the universe '), 'and everything');
     },
     get z() {
       return 42;
     },
@@ skipping 14 matching lines @@
   exports.B$ = B$;
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.compute = compute;
   exports.q = q;
   exports.z = z;
   exports.z = z;
   exports.main = main;
 })(fieldtest || (fieldtest = {}));