Conflict between builtins and mangle-names

[btmills]

(This has been moved from PR #565)

This demonstrates a conflict between babel-plugin-minify-builtins and babel-plugin-minify-mangle-names. When a property on Math or Number is referenced twice in an array literal inside a function, only the second reference gets updated with the mangled name.

// in.js
function a() {
    [Math.pi, Math.pi];
}

// out.js
function a() {
    var a = Math.pi;
    [_Mathpi, a];
}

// .babelrc
{
	"plugins": [
		"minify-builtins",
		"minify-mangle-names"
	]
}

This appears to happen only for Math or Number properties - I tried Array, Boolean, Date, and a few others, none of which had this issue.

I pushed up a runnable example to btmills/babili-mangle-builtins-conflict, so you can clone, install, and test if that's easier.