add esnext symbol proposal

src/lib/esnext.d.ts

@@ -1,3 +1,4 @@
 /// <reference path="lib.es2018.d.ts" />
 /// <reference path="lib.esnext.asynciterable.d.ts" />
 /// <reference path="lib.esnext.array.d.ts" />
+/// <reference path="lib.esnext.symbol.d.ts" />

src/lib/esnext.symbol.d.ts

@@ -0,0 +1,6 @@
+interface Symbol {
+    /**
+     * expose the [[Description]] internal slot of a symbol directly
+     */
+    readonly description: string;
+}

src/lib/libs.json

@@ -33,6 +33,7 @@
         "es2018.intl",
         "esnext.asynciterable",
         "esnext.array",
+        "esnext.symbol",
         // Default libraries
         "es5.full",
         "es2015.full",