Missing prototype functions in ES2015

There is regression after the Class implementation landed.

**Build command**:
```bash
tools/build.py --clean --debug --profile=es2015-subset
```

**Test case**:
```js
function f(a,b,c) {
  var args = Array.prototype.slice.call(arguments, 3);
  print(typeof args.splice);
}
f();
```

**Output**:
```
undefined
```

**Expected output**:
```
function
```
