[PR] Covariant length on ReadOnlyArray<.>

Summary:
$ReadOnlyArray shouldn't allow `length` setting because it can mutate the array.
Closes #4266

Reviewed By: mroch

Differential Revision: D5341152

Pulled By: nmote

fbshipit-source-id: b7de22d06d8dc5e0b73cd7c7c67db4df1ce037fe

lib/core.js

@@ -235,7 +235,7 @@ declare class $ReadOnlyArray<+T> {
     some(callbackfn: (value: T, index: number, array: $ReadOnlyArray<T>) => any, thisArg?: any): boolean;
     values(): Iterator<T>;
     +[key: number]: T;
-    length: number;
+    +length: number;
 }
 
 declare class Array<T> extends $ReadOnlyArray<T> {
@@ -275,6 +275,7 @@ declare class Array<T> extends $ReadOnlyArray<T> {
 
 
     [key: number]: T;
+    length: number;
     static (...values:Array<any>): Array<any>;
     static isArray(obj: any): bool;
     static from<A, B>(iter: Iterable<A>, mapFn: (elem: A, index: number) => B, thisArg?: any): Array<B>;

newtests/array_literal_tuple_spread/test.js

@@ -261,8 +261,8 @@ export default suite(({addFile, addFiles, addCode}) => [
           test.js:3
             3: const arr: Array<number> = [..."hello"];
                                 ^^^^^^ number. This type is incompatible with
-          290:     @@iterator(): Iterator<string>;
-                                          ^^^^^^ string. See lib: [LIB] core.js:290
+          291:     @@iterator(): Iterator<string>;
+                                          ^^^^^^ string. See lib: [LIB] core.js:291
         `,
       )
       .because('String is an Iterable<string>'),