FallbackSymbol now returns instance of self

Existing functionality is maintained using toString. This is necessary to
support type checking, and to be more consistent with the native Symbol
implementation.

Technically `new FallbackSymbol` is supposed to throw a TypeError, just as
`new Symbol` would, but doing so complicates the constructor unncessarily,
so I am not going to bother with that here.

lib/util/symbol/FallbackSymbol.js

@@ -58,9 +58,31 @@ var _root = ' ' + String.fromCharCode(
  */
 function FallbackSymbol()
 {
-    return _root + _floor( _random() * 1e8 );
+    if ( !( this instanceof FallbackSymbol ) )
+    {
+        return new FallbackSymbol();
+    }
+
+    this.___$$id$$ = ( _root + _floor( _random() * 1e8 ) );
 }
 
 
+FallbackSymbol.prototype = {
+    /**
+     * Return random identifier
+     *
+     * This is convenient, as it allows us to both treat the symbol as an
+     * object of type FallbackSymbol and use the symbol as a key (since
+     * doing so will automatically call this method).
+     *
+     * @return  {string}  random identifier
+     */
+    toString: function()
+    {
+        return this.___$$id$$;
+    },
+};
+
+
 module.exports = FallbackSymbol;
 

test/Util/symbol/FallbackSymbolTest.js

@@ -33,14 +33,14 @@ require( 'common' ).testCase(
      * to the holder of a reference to the symbol used to create that field.
      * Since this fallback is intended to be used in environments that do
      * not support symbols, the alternative is to return a random string
-     * that is highly unlikely to exist in practice.
+     * that is highly unlikely to exist in practice. However, we must also
+     * return an object to allow for instanceof checks. See below test for
+     * more details.
      */
-    'Constructor returns a generated string': function()
+    'Constructor returns an instance of Symbol': function()
     {
         var result = this.Sut();
-
+        this.assertOk( result instanceof this.Sut );
-        this.assertOk( typeof result === 'string' );
-        this.assertOk( result.length > 0 );
     },