ES6 Compilation - Internal Compiler Error

I have been working on adding a new `--allow_es6_to_es6` flag (see #780), which went well, but the compiler is tripping over itself when it reaches a class constructor.

The console output is as follows:

```
java.lang.RuntimeException: INTERNAL COMPILER ERROR.
Please report this problem.

null
Node(EXPR_RESULT): test-es6-to-es6.js:5:4
console.log('Foo');
Parent(BLOCK): test-es6-to-es6.js:4:16
constructor() {

at com.google.javascript.jscomp.OptimizeArgumentsArray.tryReplaceArguments(OptimizeArguments Array.java:182)
at com.google.javascript.jscomp.OptimizeArgumentsArray.exitScope(OptimizeArgumentsArray.java:127)
at com.google.javascript.jscomp.NodeTraversal.popScope(NodeTraversal.java:689)
```