Firestore: in tests, fix SpecWatchFilter type to work on upcoming Typescript 2.7 (#310)

* Fix definition of SpecWatchFilter:firestore tests

Previously, the type SpecWatchFilter in spec_test_runner was specified
as a tuple, but it was later used with `push` and is actually an array with a
first element that is guaranteed to be present, and of type TargetId[].

In Typescript 2.7, tuples will be fixed-length and this usage will fail.
This changes the definition of SpecWatchFilter to an interface
that extends Array<TargetId[] | string> and whose required '0' property
is a TargetId[].

Author: sandersn

packages/firestore/test/unit/specs/spec_test_runner.ts

@@ -1190,7 +1190,10 @@ export interface SpecWatchEntity {
  * Note that the last parameter is really of type ...string (spread operator)
  * The filter is based of a list of keys to match in the existence filter
  */
-export type SpecWatchFilter = [TargetId[], string];
+export interface SpecWatchFilter extends Array<TargetId[] | string> {
+  '0': TargetId[];
+  '1'?: string;
+}
 
 /**
  * [field, op, value]