Internal error when using fmt directives around a decorator

[sibsibsib]

Operating system: macos
Python version: 3.7
Black version: 19.3b0
Does also happen on master: yes [playground link]

Black outputs the following:
INTERNAL ERROR: Black produced invalid code: expected an indented block

---

to reproduce:

class A:
    # fmt: off
    @decorator(
        param=1,
        foo=2,
    )
    # fmt: on
    def __init__(self):
        pass
        

# this line will be incorrectly indented:
class B:
    # fmt: off
    @decorator(
        param=1,
        foo=2,
    )
    # fmt: on
    def __init__(self):
        pass
                

Additionally, if the second class has no formatting directives the entire class will be indented under the first one.

I'm working around this by moving the directives inside the decorator parens, eg:

@decorator(
    # fmt: off
    param=1,
    foo=2,
    # fmt: on
)

[zsol]

This is a duplicate of #560.