make StreamChannelMixin an abstract mixin class

[jakemac53]

Fixes #1947

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:stream_channel	2.1.4	ready to publish	stream_channel-v2.1.4
package:file_testing	3.1.0-wip	WIP (no publish necessary)
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)
package:cli_util	0.4.2	already published at pub.dev
package:clock	1.1.2	already published at pub.dev
package:code_builder	4.10.1	already published at pub.dev
package:coverage	1.11.1	already published at pub.dev
package:csslib	1.0.2	already published at pub.dev
package:extension_discovery	2.1.0	already published at pub.dev
package:file	7.0.2-wip	WIP (no publish necessary)
package:bazel_worker	1.1.3-wip	WIP (no publish necessary)
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:html	0.15.5	already published at pub.dev
package:io	1.0.5	already published at pub.dev
package:json_rpc_2	3.0.3	already published at pub.dev
package:mime	2.0.0	already published at pub.dev
package:oauth2	2.0.4-wip	WIP (no publish necessary)
package:package_config	2.1.1	already published at pub.dev
package:pool	1.5.2-wip	WIP (no publish necessary)
package:pub_semver	2.1.5	already published at pub.dev
package:pubspec_parse	1.5.0-wip	WIP (no publish necessary)
package:source_map_stack_trace	2.1.3-wip	WIP (no publish necessary)
package:yaml_edit	2.2.2	already published at pub.dev
package:source_span	1.10.1	already published at pub.dev
package:sse	4.1.7	already published at pub.dev
package:stack_trace	1.12.1	already published at pub.dev
package:benchmark_harness	2.3.1	already published at pub.dev
package:stream_transform	2.1.2-wip	WIP (no publish necessary)
package:string_scanner	1.4.1	already published at pub.dev
package:term_glyph	1.2.3-wip	WIP (no publish necessary)
package:test_reflective_loader	0.2.3	already published at pub.dev
package:timing	1.0.2	already published at pub.dev
package:unified_analytics	7.0.1	already published at pub.dev
package:watcher	1.1.1	already published at pub.dev
package:yaml	3.1.3	already published at pub.dev
package:source_maps	0.10.14-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
stream_channel	None	2.1.3	2.1.4	2.1.3	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage
pkgs/stream_channel/lib/stream_channel.dart	💚 100 %

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Files
pkgs/bazel_worker/benchmark/benchmark.dart
pkgs/bazel_worker/example/client.dart
pkgs/bazel_worker/example/worker.dart
pkgs/benchmark_harness/integration_test/perf_benchmark_test.dart
pkgs/boolean_selector/example/example.dart
pkgs/clock/lib/clock.dart
pkgs/clock/lib/src/clock.dart
pkgs/clock/lib/src/default.dart
pkgs/clock/lib/src/stopwatch.dart
pkgs/clock/lib/src/utils.dart
pkgs/clock/test/clock_test.dart
pkgs/clock/test/default_test.dart
pkgs/clock/test/stopwatch_test.dart
pkgs/clock/test/utils.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/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