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Using a trusted type policy for the script elements injected to load deferred libraries allows a content security policy to be enabled to prevent untrusted scripts elements from loading code. TEST=https://dart-review.googlesource.com/c/sdk/+/237638 Change-Id: Ie3505a68d83cb19c7f8bdbefa01df7fc58d9924a Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237542 Reviewed-by: Sigmund Cherem <sigmund@google.com> Commit-Queue: Stephen Adams <sra@google.com>