build(core): remove main() from specs (#21053)

`main()` function used to be needed to support dart, since dart
Does not allow top level statements. Since we no longer use dart
The need for `main()` has been removed.

In preparation for `Basel` and standardized way of running tests
we are removing `main()`

PR Close #21053

packages/core/test/util/decorators_spec.ts

@@ -12,7 +12,7 @@ import {ANNOTATIONS, makeDecorator, makePropDecorator} from '../../src/util/deco
 class DecoratedParent {}
 class DecoratedChild extends DecoratedParent {}
 
-export function main() {
+{
   const TerminalDecorator =
       makeDecorator('TerminalDecorator', (data: any) => ({terminal: true, ...data}));
   const TestDecorator = makeDecorator(

packages/core/test/util/lang_spec.ts

@@ -8,7 +8,7 @@
 import {isObservable, isPromise} from '@angular/core/src/util/lang';
 import {of } from 'rxjs/observable/of';
 
-export function main() {
+{
   describe('isPromise', () => {
     it('should be true for native Promises',
        () => expect(isPromise(Promise.resolve(true))).toEqual(true));