Allow LHS in destructuring-assignment

[alexzherdev]

Resolves #1728

Not sure if this needs to also check that assignment is in the constructor? Also, would the following do anything useful / should this be allowed?

constructor(props, context) {
  this.context.foo = 'blah';
}

[ljharb]

this test case doesn't require babel-eslint, let's remove this line

[EvHaus]

LGTM