False positive "property update already defined on interface" when extern contains untyped property called "update"

Mailing list link to thread that spawned this bug report: https://groups.google.com/d/topic/closure-compiler-discuss/0JAsjIFc5Rs/discussion

Closure outputs false positive messages `WARNING - property update already defined on interface ; use @override to override it` when compiling with an extern that defines an untyped symbol called `update`. Changing the name of the symbol from `update` to anything else seems to fix the false positive. Adding type annotations to the symbol also seems to fix the false positive.

Compiler version latest:
```
$ cat /usr/local/bin/closure-compiler
#!/bin/bash
exec java -jar /usr/local/lib/closure/latest/compiler.jar "$@"
$ /usr/local/bin/closure-compiler --version
Closure Compiler (http://github.com/google/closure-compiler)
Version: v20190301
Built on: 2019-03-05 22:03
```

Repro:
```
$ cat externs.js
/** @type {!Object} */
let SomeLibrary;

/** @type {!Object} */
SomeLibrary.someThing;

SomeLibrary.someThing.update;

$ cat empty.js

$ /usr/local/bin/closure-compiler --externs externs.js --js empty.js --jscomp_warning '*'
externs.zip//html5.js:1268: WARNING - property update already defined on interface EventTarget; use @override to override it
DOMApplicationCache.prototype.update = function() {};
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

0 error(s), 1 warning(s)
```

This is a false positive as `EventTarget` does not define any symbol called `update`.

In the above example, either changing `SomeLibrary.someThing.update` to be called anything else (e.g. `SomeLibrary.someThing.somethingElse`) or adding a type annotation to it (e.g. `/** @type {!Object} */`) causes this false positive to go away.

These false positive errors continue to seemingly any interface used in code:
```
$ cat example.js
/**
* @interface
*/
class MyInterface {
}

/**
* @implements {MyInterface}
*/
class MyClass {
/**
* Does something.
*/
update() {
}
}

$ /usr/local/bin/closure-compiler --externs externs.js --js example.js --jscomp_warning '*'
externs.zip//html5.js:1268: WARNING - property update already defined on interface EventTarget; use @override to override it
DOMApplicationCache.prototype.update = function() {};
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

example.js:14: WARNING - property update already defined on interface MyInterface; use @override to override it
update() {
^^^^^^^^^^

0 error(s), 2 warning(s), 100.0% typed
```

The `MyInterface` interface clearly does not define a method called `update`.