perf(core): Remove decorator DSL which depends on Reflect

BREAKING CHANGE

It is no longer possible to declare classes in this format.

```
Component({...}).
Class({
  constructor: function() {...}
})
```

This format would only work with JIT and with ES5. This mode doesn’t
allow build tools like Webpack to process and optimize the code, which
results in prohibitively large bundles. We are removing this API
because we are trying to ensure that everyone is on the fast path by
default, and it is not possible to get on the fast path using the ES5
DSL. The replacement is to use TypeScript and `@Decorator` format.

```
@Component({...})
class {
  constructor() {...}
}
```

packages/core/src/core.ts

@@ -13,7 +13,7 @@
  */
 export * from './metadata';
 export * from './version';
-export {Class, ClassDefinition, TypeDecorator} from './util/decorators';
+export {TypeDecorator} from './util/decorators';
 export * from './di';
 export {createPlatform, assertPlatform, destroyPlatform, getPlatform, PlatformRef, ApplicationRef, enableProdMode, isDevMode, createPlatformFactory, NgProbeToken} from './application_ref';
 export {APP_ID, PACKAGE_ROOT_URL, PLATFORM_INITIALIZER, PLATFORM_ID, APP_BOOTSTRAP_LISTENER} from './application_tokens';