Infer Null for return type of functions with empty returns.

pkg/dev_compiler/test/codegen_expected/closure.js

 export const closure = Object.create(null);
 import { core, js, dart, dartx } from 'dart_sdk';
 let dynamic__Toint = () => (dynamic__Toint = dart.constFn(dart.functionType(core.int, [dart.dynamic], [dart.dynamic])))();
 let dynamic__Todynamic = () => (dynamic__Todynamic = dart.constFn(dart.functionType(dart.dynamic, [dart.dynamic], {y: core.String, z: dart.dynamic})))();
 let StringToList = () => (StringToList = dart.constFn(dart.functionType(core.List, [core.String])))();
 let ListOfint = () => (ListOfint = dart.constFn(core.List$(core.int)))();
 let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.functionType(dart.dynamic, [dart.dynamic])))();
 let FnToListOfint = () => (FnToListOfint = dart.constFn(dart.functionType(ListOfint(), [dynamicTodynamic()])))();
 let MapOfMap$Map = () => (MapOfMap$Map = dart.constFn(core.Map$(core.Map, core.Map)))();
 let Foo = () => (Foo = dart.constFn(closure.Foo$()))();
 let ListOfTAndTToListOfT = () => (ListOfTAndTToListOfT = dart.constFn(dart.definiteFunctionType(T => [core.List$(T), [core.List$(T), T]])))();
 let dynamicTovoid = () => (dynamicTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [dart.dynamic])))();
-let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+let VoidToNull = () => (VoidToNull = dart.constFn(dart.definiteFunctionType(core.Null, [])))();
 closure.generic_function = function(T) {
   return (items: core.List<T>, seed: T): core.List<T> => {
     let strings = items[dartx.map](core.String)(dart.fn((i: T): string => dart.str`${i}`, dart.definiteFunctionType(core.String, [T])))[dartx.toList]();
     return items;
   };
 };
 dart.fn(closure.generic_function, ListOfTAndTToListOfT());
 closure.Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
 closure.Foo$ = dart.generic(T => {
   let FooOfT = () => (FooOfT = dart.constFn(closure.Foo$(T)))();
@@ skipping 88 matching lines @@
 };
 dart.addSimpleTypeTests(closure.Baz);
 dart.setSignature(closure.Baz, {
   constructors: () => ({new: dart.definiteFunctionType(closure.Baz, [core.int])})
 });
 closure.main = function(args): void {
 };
 dart.fn(closure.main, dynamicTovoid());
 dart.defineLazy(closure, {
   get closure() {
-    return dart.fn(() => {
+    return dart.fn((): core.Null => {
       return;
-    }, VoidTodynamic());
+    }, VoidToNull());
   },
   set closure(_) {}
 });
 /** @final {string} */
 closure.some_top_level_constant = "abc";
 /** @final {string} */
 closure.some_top_level_final = "abc";
 /** @type {string} */
 closure.some_top_level_var = "abc";