[PR] Fix static declarations for XMLHttpRequest

Summary:
Encountered this error while using static values from [XMLHttpRequest](https://xhr.spec.whatwg.org/#interface-xmlhttprequest):
```
Cannot get `XMLHttpRequest.DONE` because property `DONE` is missing in statics of `XMLHttpRequest`
```
I think they were never declared properly in the [bom.js](https://github.com/facebook/flow/blob/master/lib/bom.js#L584) file. Here's a [stripped down tryflow](https://flow.org/try/#0CYUwxgNghgTiAEkoGdnwMIFcZwHYBcANAWQBkAJffABwCUQBHTEZfedgbwCh32ARAPIA5AKIAueLkwBbAEYgYAbi49erKPgCWYZBO68DiOBpAAKAJQSsOEARIUqdRs1bKAkAcGiJUuQuUGAL5cwVygSHCI0KjwAAowAPbUCcggwABimgAe9pQ09EwsbJxcbupaYPBe4pIy8kqq-MI1vvXKjfDl2rrw+obsYMb4ZpZxicmpGdm5jgUu+AG8waFgCbis8MOsAIzwALwY2HhEZHlOhawAdNVcq+tsW-gATPtjSSlpmTmns85F180gA) demonstrating the issue as well as the fix.

On a related note, I'm not sure what the `create()` declaration in the `statics` block is for either.
Pull Request resolved: #6091

Reviewed By: dsainati1

Differential Revision: D13820105

Pulled By: nmote

fbshipit-source-id: 624042d694e2804473e75751bf75c61d9d21f65a

Author: robin-pham

lib/bom.js

@@ -579,6 +579,11 @@ declare class XDomainRequest {
 
 
 declare class XMLHttpRequest extends EventTarget {
+    static LOADING: number;
+    static DONE: number;
+    static UNSENT: number;
+    static OPENED: number;
+    static HEADERS_RECEIVED: number;
     responseBody: any;
     status: number;
     readyState: number;
@@ -616,12 +621,6 @@ declare class XMLHttpRequest extends EventTarget {
 
     statics: {
         create(): XMLHttpRequest;
-
-        LOADING: number;
-        DONE: number;
-        UNSENT: number;
-        OPENED: number;
-        HEADERS_RECEIVED: number;
     }
 }
 

tests/bom/bom.exp

@@ -578,8 +578,8 @@ Cannot assign `headers.get(...)` to `b` because null [1] is incompatible with st
                           ^^^^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/bom.js:951:24
-   951|     get(name: string): null | string;
+   <BUILTINS>/bom.js:950:24
+   950|     get(name: string): null | string;
                                ^^^^ [1]
    Headers.js:8:10
      8| const b: string = headers.get('foo'); // incorrect
@@ -803,8 +803,8 @@ Cannot assign `params.get(...)` to `b` because null [1] is incompatible with str
                           ^^^^^^^^^^^^^^^^^
 
 References:
-   <BUILTINS>/bom.js:965:24
-   965|     get(name: string): null | string;
+   <BUILTINS>/bom.js:964:24
+   964|     get(name: string): null | string;
                                ^^^^ [1]
    URLSearchParams.js:8:10
      8| const b: string = params.get('foo'); // incorrect