Add external factories for @staticInterop classes instead of synthetic constructor

[srujzs]

Per dart-lang/sdk#49941, synthetic constructors will be disallowed on @staticInterop classes. Functionally, they are lowered to the same code.

Also fixes an erroneous call to the DomHTMLStyleConstructor instead of the relevant factory method.

Migration needed to fix dart-lang/sdk#49941

These factories were added because they were caught by the flutter trybots (https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8798044738469408369/+/u/build_host_debug/stdout).

Pre-launch Checklist

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• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
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• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

[skia-gold]

Gold has detected about 103 new digest(s) on patchset 1.
View them at https://flutter-engine-gold.skia.org/cl/github/37443

[yjbanov]

[flutter-dashboard]

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Changes reported for pull request #37443 at sha bf4ad66

[Hixie]

test-exemption: code refactor with no semantic change

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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