fix(platform-browser): update type castings for JSON.parse usage (angular#40710)

Update usages of JSON.parse to be cast as specific types.

PR Close angular#40710

Author: josephperrott

packages/platform-browser/test/browser/transfer_state_spec.ts

@@ -127,7 +127,7 @@ describe('escape/unescape', () => {
         '{&q;testString&q;:&q;&l;/script&g;&l;script&g;' +
         'alert(&s;Hello&a;&s; + \\&q;World\\&q;);&q;}');
 
-    const unescapedObj = JSON.parse(unescapeHtml(escaped));
+    const unescapedObj = JSON.parse(unescapeHtml(escaped)) as {testString: string};
     expect(unescapedObj['testString']).toBe(testString);
   });
 });