Protect the program's `global` Object
Protect the program's `global` Object

I assume the intent of the sandbox is _not_ to allow code inside the sandbox to pollute the program's `global` and allow modifications to it.

Compare existing:
```
A = Object.assign(global,{z:4})
global.z // 4
A.foo = 5
global.foo // 5
```
with
```
A = Object.assign({},global,{z:4})
global.z // undefined
A.foo = 5
global.foo // undefined
```
DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax,DmitrySoshnikov/syntax