Make int and double nullable by default

test/codegen/expect/constructors.js

 var constructors;
 (function(exports) {
   'use strict';
   class A extends dart.Object {
   }
   class B extends dart.Object {
     B() {
     }
   }
   class C extends dart.Object {
@@ skipping 59 matching lines @@
     }
   }
   dart.defineNamedConstructor(K, 'withS');
   class L extends dart.Object {
     L(foo) {
       this.foo = foo;
     }
   }
   class M extends L {
     M$named(x) {
-      super.L(x + 42);
+      super.L(dart.notNull(x) + 42);
     }
   }
   dart.defineNamedConstructor(M, 'named');
   class N extends M {
     N$named(y) {
-      super.M$named(y + 100);
+      super.M$named(dart.notNull(y) + 100);
     }
   }
   dart.defineNamedConstructor(N, 'named');
   class P extends N {
     P(z) {
-      super.N$named(z + 9000);
+      super.N$named(dart.notNull(z) + 9000);
     }
     P$foo(x) {
-      this.P(x + 42);
+      this.P(dart.notNull(x) + 42);
     }
     P$bar() {
       this.P$foo(1);
     }
   }
   dart.defineNamedConstructor(P, 'foo');
   dart.defineNamedConstructor(P, 'bar');
   // Exports:
   exports.A = A;
   exports.B = B;
   exports.C = C;
   exports.C2 = C2;
   exports.D = D;
   exports.E = E;
   exports.F = F;
   exports.G = G;
   exports.H = H;
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
 })(constructors || (constructors = {}));