Improve typing of Symbol.iterator

Summary: Previously, `Symbol.iterator` was just typed as `string`. Now, it is typed with the polyfilled value (that we also use internally). Maybe one day in the future we can actually use symbols as property keys, but until then this is an improvement from the previous state.

Reviewed By: mroch

Differential Revision: D27611562

fbshipit-source-id: bf31193eb353a532dbeb2eb1ba397fb57fe18097

lib/core.js

@@ -278,7 +278,7 @@ declare class Symbol {
    * by Array.prototype.concat.
    */
   static isConcatSpreadable: $SymboIsConcatSpreadable;
-  static iterator: string; // polyfill '@@iterator'
+  static iterator: '@@iterator'; // polyfill '@@iterator'
   /**
    * Returns a key from the global symbol registry matching the given Symbol if found.
    * Otherwise, returns a undefined.

tests/symbol/iterator.js

@@ -0,0 +1,6 @@
+declare var foo: {
+  @@iterator: Iterator<number>,
+};
+
+(foo[Symbol.iterator]: Iterator<number>); // OK
+(foo[Symbol.iterator]: boolean); // Error

tests/symbol/symbol.exp

@@ -1,3 +1,21 @@
+Error -------------------------------------------------------------------------------------------------- iterator.js:6:2
+
+Cannot cast `foo[Symbol.iterator]` to boolean because `$Iterator` [1] is incompatible with boolean [2].
+[incompatible-cast]
+
+   iterator.js:6:2
+   6| (foo[Symbol.iterator]: boolean); // Error
+       ^^^^^^^^^^^^^^^^^^^^
+
+References:
+   iterator.js:2:15
+   2|   @@iterator: Iterator<number>,
+                    ^^^^^^^^^^^^^^^^ [1]
+   iterator.js:6:24
+   6| (foo[Symbol.iterator]: boolean); // Error
+                             ^^^^^^^ [2]
+
+
 Error --------------------------------------------------------------------------------------------------- symbol.js:8:17
 
 Cannot call `Symbol` because no more than 1 argument is expected by function type [1]. [extra-arg]
@@ -137,4 +155,4 @@ References:
 
 
 
-Found 8 errors
+Found 9 errors