Add null checks for CanvasElement attributes

[srujzs]

Changes in processing compatibility info in dart:html requires
these getters to be null-checked.

Description

CanvasElement attributes will be changed to be potentially nullable (see https://dart-review.googlesource.com/c/sdk/+/155844), so they are null-checked.

Related Issues

No new issue for this, but related to the original process in dart-lang/sdk#41905.

Tests

No tests added.

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Breaking Change

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• No, no existing tests failed, so this is not a breaking change.

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[gaaclarke]

Check out the "format_and_dart_test" Looks like you are using non-nullable features in code that isn't compiling with them.

[srujzs]

This is strange. In my local build, when I add the aforementioned Dart SDK patch to third_party and attempt to felt build without this CL and enforcing null-check lints, I see null-check related errors e.g. Error: Operator '*' cannot be called on 'int?' because it is potentially null. I'm not sure if the Flutter try bots are configured improperly on the Dart SDK, but I'm seeing similar errors: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8872137059857262464/+/steps/build_host_debug/0/stdout

Although, now when I run felt test rebased to the latest, I'm seeing the same error as https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8871672762064432336/+/steps/felt_ios-safari_test/0/stdout

[gaaclarke]

Paging @yjbanov, he knows more about web_ui and the current state of the dart null safety code.

[yjbanov]

Windows host engine failure looks unrelated (infra flake even). Merging.