Fix the method used to determine whether Symbol is polyfilled.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131721268

test/com/google/javascript/jscomp/runtime_tests/polyfill_tests/object_getownpropertysymbols_test.js

@@ -23,7 +23,7 @@ const testing = goog.require('jscomp.runtime_tests.polyfill_tests.testing');
 const assertPropertyListEquals = testing.assertPropertyListEquals;
 const objectCreate = testing.objectCreate;
 
-const SYMBOL_IS_POLYFILLED = typeof Symbol('') === 'string';
+const SYMBOL_IS_POLYFILLED = Symbol != $native.Symbol;
 
 testSuite({
   testGetOwnPropertySymbols_symbols() {