$a can override a property/function set by the application

[austinrmiller]

Augury version (required): 1.4.0
Angular version (required): 4.0.0
Date: 3/29/2017
OS: Mac

Description of issue:
Augury has this code in backend.ts:

const selectedComponentPropertyKey = '$a';

const noSelectedComponentWarningText = 'There is no component selected.';

Object.defineProperty(window, selectedComponentPropertyKey,
  {value: noSelectedComponentWarningText});

But that can easily override a $a property/function used by the application. This is especially likely when using Closure Compiler since it will frequently rename a function to $a, and sometimes that ends up being a function on window. I work at Google where I ran into this.

Steps to reproduce:

1. Define window.$a to be a function that is called at runtime by the main application
2. Load the page with Augury installed
3. Get error