Infer Null for return type of functions with empty returns.

pkg/dev_compiler/test/codegen_expected/BenchmarkBase.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const BenchmarkBase$ = Object.create(null);
-  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  let VoidToNull = () => (VoidToNull = dart.constFn(dart.definiteFunctionType(core.Null, [])))();
   BenchmarkBase$.Expect = class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
         dart.throw(dart.str`Values not equal: ${expected} vs ${actual}`);
       }
     }
     static listEquals(expected, actual) {
       if (expected[dartx.length] != actual[dartx.length]) {
         dart.throw(dart.str`Lists have different lengths: ${expected[dartx.length]} vs ${actual[dartx.length]}`);
       }
@@ skipping 38 matching lines @@
         dart.dcall(f);
         elapsed = watch.elapsedMilliseconds;
         iter++;
       }
       return 1000.0 * dart.notNull(elapsed) / iter;
     }
     measure() {
       this.setup();
       BenchmarkBase$.BenchmarkBase.measureFor(dart.fn(() => {
         this.warmup();
-      }, VoidTodynamic()), 100);
+      }, VoidToNull()), 100);
       let result = BenchmarkBase$.BenchmarkBase.measureFor(dart.fn(() => {
         this.exercise();
-      }, VoidTodynamic()), 2000);
+      }, VoidToNull()), 2000);
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
       core.print(dart.str`${this.name}(RunTime): ${score} us.`);
     }
   };
   dart.setSignature(BenchmarkBase$.BenchmarkBase, {
     constructors: () => ({new: dart.definiteFunctionType(BenchmarkBase$.BenchmarkBase, [core.String])}),
     fields: () => ({name: core.String}),
     methods: () => ({
       run: dart.definiteFunctionType(dart.void, []),
       warmup: dart.definiteFunctionType(dart.void, []),
       exercise: dart.definiteFunctionType(dart.void, []),
       setup: dart.definiteFunctionType(dart.void, []),
       teardown: dart.definiteFunctionType(dart.void, []),
       measure: dart.definiteFunctionType(core.double, []),
       report: dart.definiteFunctionType(dart.void, [])
     }),
     statics: () => ({measureFor: dart.definiteFunctionType(core.double, [core.Function, core.int])}),
     names: ['measureFor']
   });
   // Exports:
   return {
     BenchmarkBase: BenchmarkBase$
   };
 });