Allow mixed args to string.prototype.concat()

According to MDN docs for the `concat()` method of the `String` type: 

> If the arguments are not of the type string, they are converted to string
> values before concatenating.

Passing numbers, objects, etc to `String.prototype.concat` is is valid JS
behavior but causes an error in Flow. It's also blocking 
facebook/react#22064.

This commit changes the arg type: `Array<string>` -> `Array<mixed>`.

lib/core.js

@@ -1103,9 +1103,11 @@ declare class String {
     codePointAt(index: number): number;
     /**
      * Returns a string that contains the concatenation of two or more strings.
-     * @param strings The strings to append to the end of the string.
+     * If the arguments are not of the type string, they are converted to string values before
+     * concatenating.
+     * @param values The values to append to the end of the string.
      */
-    concat(...strings: Array<string>): string;
+    concat(...values: Array<mixed>): string;
     constructor(value?: mixed): void;
     /**
      * Returns true if the sequence of elements of searchString converted to a String is the