Compiler warnings when referencing inherited non-public property in getter/setter

Given:

```js
// a.js

export class A {
constructor() {
/** @protected {string} */
this.protected_ = '';
}
}
```

```js
// b.js

import {A} from './a';

class B extends A {
constructor() {
super();
/** @const {string} */
this.publicProp;
}

/**
* @return {string}
*/
get publicProp() {
return this.protected_;
}

/** @return {string} */
getPublicProp() {
return this.protected_;
}
}

const b = new B();
b.publicProp = '?';
console.log(b.publicProp);
```

Running:

```sh
java -jar ../node_modules/google-closure-compiler/compiler.jar --compilation_level ADVANCED \
--js *.js \
--language_out ECMASCRIPT5_STRICT \
--entry_point b.js \
--checks_only
```

Yields:

```
b.js:14: WARNING - Access to protected property protected_ of B not allowed here.
return this.protected_;
^^^^^^^^^^^^^^^

0 error(s), 1 warning(s), 94.0% typed
```

Note how `this.protected_` could be accessed within the regular method, but not within the getter/setter.

Discovered this while doing work for https://github.com/material-components/material-components-web/issues/134. We make heavy use of getters and setters due to our desire to align with DOM APIs for native elements, so I'd be willing to help fix this if someone could point me in the right direction :)