Redundant unary '+' operators not merged

asm.js, from a strictly JavaScript perspective, produces a lot of redundant operators. For example, a function that takes a double and returns the same double would be expressed as:

``` javascript
function f(x) {
x = +x; // tells asm.js that the argument is a double
return +x;
}
```

Closure Compiler minifies that into:

``` javascript
function f(a){return+ +a}
```

Of course, the double + operator isn't necessary.