adding html canvas example

examples/html_canvas/.gitignore

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+# https://dart.dev/guides/libraries/private-files
+# Created by `dart pub`
+.dart_tool/

examples/html_canvas/CHANGELOG.md

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+## 1.0.0
+
+- Initial version.

examples/html_canvas/README.md

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+A bare-bones Dart web app.
+
+Uses [`package:web`](https://pub.dev/packages/web)
+to interop with JS and the DOM.
+
+## Running and building
+
+To run the app, use these commands:
+```
+dart pub global activate webdev
+webdev serve
+```
+
+To build a production version ready for deployment, use these commands:
+```
+webdev build
+```
+
+For more details, see https://dart.dev/web/get-started

examples/html_canvas/analysis_options.yaml

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+# This file configures the static analysis results for your project (errors,
+# warnings, and lints).
+#
+# This enables the 'recommended' set of lints from `package:lints`.
+# This set helps identify many issues that may lead to problems when running
+# or consuming Dart code, and enforces writing Dart using a single, idiomatic
+# style and format.
+#
+# If you want a smaller set of lints you can change this to specify
+# 'package:lints/core.yaml'. These are just the most critical lints
+# (the recommended set includes the core lints).
+# The core lints are also what is used by pub.dev for scoring packages.
+
+include: package:lints/recommended.yaml
+
+# Uncomment the following section to specify additional rules.
+
+# linter:
+#   rules:
+#     - camel_case_types
+
+# analyzer:
+#   exclude:
+#     - path/to/excluded/files/**
+
+# For more information about the core and recommended set of lints, see
+# https://dart.dev/go/core-lints
+
+# For additional information about configuring this file, see
+# https://dart.dev/guides/language/analysis-options