Mutable exports lead to incorrect code removal

Given these two files ([service repro](https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540language_out%2520ES5%250A%252F%252F%2520%2540code_url%2520http%253A%252F%252Fpersistent.info%252Ftmp%252Fbase.js%250A%252F%252F%2520%2540code_url%2520http%253A%252F%252Fpersistent.info%252Ftmp%252Fmain.js%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A)):
```js
// base.js
export function initialize(fn) {
fn();
}

function reassignInitialize(wrapper) {
initialize = function(fn) {
fn();
};
}

// main.js
import * as base from "./base.js";

console.log("Loaded main");

export function initialize() {
console.log("Running initialize");
}

base.initialize(initialize);
```

The expected output is `console.log("Loaded main");console.log("Running initialize")`. However, only `console.log("Loaded main");` appears.

If we drop the `reassignInitialize` function ([service repro](https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540language_out%2520ES5%250A%252F%252F%2520%2540code_url%2520http%253A%252F%252Fpersistent.info%252Ftmp%252Fbase-no-reassignment.js%250A%252F%252F%2520%2540code_url%2520http%253A%252F%252Fpersistent.info%252Ftmp%252Fmain-no-reassignment.js%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A)) then it works as expected.

I'm guessing that something in the mutable export implementation (679ea8fb4e8808ee725d22b8156ec09c96e437b0 - it uses getters) is not playing well with advanced optimizations, but I have not investigated further.

Run the linked Closure Compiler repro with base.js and main.js under ADVANCED_OPTIMIZATIONS, then compare it with the no-reassignment variant. Trace the mutable export implementation introduced by commit 679ea8fb4e8808ee725d22b8156ec09c96e437b0. Done means the original repro preserves both expected console.log calls.

Written by the indexing model from the issue text.