Infer Null for return type of functions with empty returns.

pkg/dev_compiler/test/codegen_expected/closure.js

Index: pkg/dev_compiler/test/codegen_expected/closure.js
diff --git a/pkg/dev_compiler/test/codegen_expected/closure.js b/pkg/dev_compiler/test/codegen_expected/closure.js
index 698b90fcbb881afeaab3919bc27654943bed1837..3db08c0e8501b2b20dce11b2e8e26d92530918a9 100644
--- a/pkg/dev_compiler/test/codegen_expected/closure.js
+++ b/pkg/dev_compiler/test/codegen_expected/closure.js
@@ -10,7 +10,7 @@ let MapOfMap$Map = () => (MapOfMap$Map = dart.constFn(core.Map$(core.Map, core.M
 let Foo = () => (Foo = dart.constFn(closure.Foo$()))();
 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]();
@@ -119,9 +119,9 @@ 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(_) {}
 });