Improve usability of TrustedTypes API

[ditman]

TrustedTypes definitions have landed, but please consider the following changes to the API to make it more usable:

1. Make trustedTypes nullable, or provide a way to check if trustedTypes are available. TrustedTypes is a Chrome-only feature, and is undefined when not available. This is what I needed to do the last time I used TT:
   • https://github.com/flutter/packages/blob/dcbaee53897d02a2511dbed593595dabc4e37310/packages/google_identity_services_web/lib/src/js_interop/package_web_tweaks.dart#L11-L17
2. Also, it's needed for certain types to have setters that accept their TrustedType value, like: script.src and element.innerHtml (and have: script.trustedSrc and element.trustedInnerHtml...), for example:
   • https://github.com/flutter/packages/blob/dcbaee53897d02a2511dbed593595dabc4e37310/packages/google_identity_services_web/lib/src/js_interop/package_web_tweaks.dart#L29-L35
3. Also also, the second parameter to createScriptURL (and probably all other create functions) must be optional. I needed to override the method last time I used this:
   • https://github.com/flutter/packages/blob/dcbaee53897d02a2511dbed593595dabc4e37310/packages/google_identity_services_web/lib/src/js_interop/package_web_tweaks.dart#L19-L27

/cc @kevmoo

TL;DR: Take this, fill in the gaps and make it part of package:web ppppplease :P