Make RelayObservable Source return type disjoint

Summary:
In Flow v0.99 we are changing function type annotations to be strict about their
static properties. This causes a small issue with the union of a Subcription and
`() => mixed` function type, where the latter is now understood to possibly have
an `unsubscribe` property with a mixed type.

This causes the following refinement check, which appears lower in this file, to
cause an error in the next version of Flow:

```
if (cleanup) {
  if (cleanup.unsubscribe) {
    cleanup.unsubscribe(); // <-- error here
  }
  // ...
}
```

In Flow v0.99, because `() => mixed` statics are now checked, Flow sees that
`cleanup.unsubscribe` might be `mixed`, which passes the conditional but could
cause an exception when called.

I also needed to change JestMockFn to have exact statics, because tests
sometimes use a value with that type in place of the cleanup function. That runs
into the `{} <: {+p?: void}` rule, which is an error. `{||} <: {+p?:void}` is
not an error.

Reviewed By: josephsavona

Differential Revision: D15522655

fbshipit-source-id: 2ae3c9016e2b07abaac79827082d2f8743623eb5

flow/jest.js

@@ -11,7 +11,7 @@
 
 /* eslint-disable lint/no-unclear-flowtypes */
 
-type JestMockFn<TArguments: $ReadOnlyArray<*>, TReturn> = {
+type JestMockFn<TArguments: $ReadOnlyArray<mixed>, TReturn> = {|
   (...args: TArguments): TReturn,
   /**
    * An object for introspecting mock calls
@@ -105,7 +105,7 @@ type JestMockFn<TArguments: $ReadOnlyArray<*>, TReturn> = {
    * Sugar for jest.fn().mockImplementationOnce(() => Promise.reject(value))
    */
   mockRejectedValueOnce(value: TReturn): JestMockFn<TArguments, Promise<any>>,
-};
+|};
 
 type JestAsymmetricEqualityType = {
   /**
@@ -794,7 +794,7 @@ type JestObjectType = {
    * Returns a new, unused mock function. Optionally takes a mock
    * implementation.
    */
-  fn<TArguments: $ReadOnlyArray<*>, TReturn>(
+  fn<TArguments: $ReadOnlyArray<mixed>, TReturn>(
     implementation?: (...args: TArguments) => TReturn,
   ): JestMockFn<TArguments, TReturn>,
   /**