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 currently causes an error in Flow. It also may be 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

newtests/autocomplete/test.js

@@ -158,7 +158,7 @@ export default (suite(({addFile, flowCmd}) => [
              },
              {
                "name": "concat",
-               "type": "(...strings: Array<string>) => string"
+               "type": "(...values: Array<mixed>) => string"
              },
              {
                "name": "endsWith",

tests/autocomplete/autocomplete.exp

@@ -41,7 +41,7 @@ Flags: --pretty
     {"name":"charAt","type":"(pos: number) => string"},
     {"name":"charCodeAt","type":"(index: number) => number"},
     {"name":"codePointAt","type":"(index: number) => number"},
-    {"name":"concat","type":"(...strings: Array<string>) => string"},
+    {"name":"concat","type":"(...values: Array<mixed>) => string"},
     {
       "name":"endsWith",
       "type":"(searchString: string, position?: number) => boolean"
@@ -389,7 +389,7 @@ Flags: --pretty
     {"name":"charAt","type":"(pos: number) => string"},
     {"name":"charCodeAt","type":"(index: number) => number"},
     {"name":"codePointAt","type":"(index: number) => number"},
-    {"name":"concat","type":"(...strings: Array<string>) => string"},
+    {"name":"concat","type":"(...values: Array<mixed>) => string"},
     {
       "name":"endsWith",
       "type":"(searchString: string, position?: number) => boolean"