[oauth2] Add isClosed and exception handling for closed state to oaut…

[illaz]

Until now there is no way to detect whether an oauth2.Client is closed or not. This PR changes that.

There is now an isClosed field on the oauth2.Client.

A request on a closed client no longer leads to a 'Null check operator used on a null value' error, but instead an http.ClientException is thrown with a meaningful error message.

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[sigurdm]

Should we just not set httpClient to null in close()? In that way we automatically inherit the close() behavior of the wrapped client...

[illaz]

Should we just not set httpClient to null in close()? In that way we automatically inherit the close() behavior of the wrapped client...

Would also be fine for me. This would at least avoid the 'Null check operator used on a null value' error.
I can also implement the isClosed behavior in my own code. It does not have to be included in this PR.

Is it okay to make a fixup commit here? Or should I just amend my existing commit?

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:bazel_worker	1.1.3	already published at pub.dev
package:benchmark_harness	2.3.1	already published at pub.dev
package:boolean_selector	2.1.2	already published at pub.dev
package:browser_launcher	1.1.3	already published at pub.dev
package:cli_config	0.2.1-wip	WIP (no publish necessary)
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package:stack_trace	1.12.1	already published at pub.dev
package:stream_channel	2.1.4	already published at pub.dev
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[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
oauth2	Non-Breaking	2.0.3	2.0.4-wip	2.1.0 Got "2.0.4-wip" expected >= "2.1.0" (non-breaking changes)	⚠️

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Changelog Entry ❗

Package	Changed Files
package:oauth2	pkgs/oauth2/lib/src/client.dart

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pkgs/oauth2/lib/src/client.dart	💚 95 % ⬆️ 10 %

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pkgs/bazel_worker/benchmark/benchmark.dart
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pkgs/coverage/lib/src/coverage_options.dart
pkgs/coverage/test/collect_coverage_config_test.dart
pkgs/coverage/test/config_file_locator_test.dart
pkgs/html/example/main.dart
pkgs/html/lib/dom.dart
pkgs/html/lib/dom_parsing.dart
pkgs/html/lib/html_escape.dart
pkgs/html/lib/parser.dart
pkgs/html/lib/src/constants.dart
pkgs/html/lib/src/encoding_parser.dart
pkgs/html/lib/src/html_input_stream.dart
pkgs/html/lib/src/list_proxy.dart
pkgs/html/lib/src/query_selector.dart
pkgs/html/lib/src/token.dart
pkgs/html/lib/src/tokenizer.dart
pkgs/html/lib/src/treebuilder.dart
pkgs/html/lib/src/utils.dart
pkgs/html/test/dom_test.dart
pkgs/html/test/parser_feature_test.dart
pkgs/html/test/parser_test.dart
pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
pkgs/html/test/selectors/selectors.dart
pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/html/test/trie_test.dart
pkgs/html/tool/generate_trie.dart
pkgs/pubspec_parse/test/git_uri_test.dart
pkgs/stack_trace/example/example.dart
pkgs/watcher/test/custom_watcher_factory_test.dart
pkgs/yaml_edit/example/example.dart