add StandardMessageCodec extracted from Flutter SDK (flutter#2434)

packages/standard_message_codec/.gitignore

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+# Libraries should not include pubspec.lock, per https://dart.dev/guides/libraries/private-files#pubspeclock.
+/pubspec.lock
+**/doc/api/
+.dart_tool/
+.packages
+build/

packages/standard_message_codec/AUTHORS

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+# Below is a list of people and organizations that have contributed
+# to the Flutter project. Names should be added to the list like so:
+#
+#   Name/Organization <email address>
+
+Google Inc.

packages/standard_message_codec/CHANGELOG.md

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+## 0.0.1
+
+* Initial release of standard message codec extracted from the Flutter SDK.

packages/standard_message_codec/LICENSE

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+Copyright 2013 The Flutter Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/standard_message_codec/README.md

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+An efficient and schemaless binary format used by the Flutter SDK.
+
+## Features
+
+### Efficiency
+
+The standard message codec is a binary format, as opposed to text based formats
+like JSON. Consider the following snippet of JSON:
+
+```json
+{
+    "data": [1, 2, 3, 4],
+}
+```
+
+In order for this message to be decoded into a Dart map, a utf8 binary file must
+first be parsed and validated into a Dart string. Then a second pass is performed
+which looks for specific characters that indicate JSON structures - for example
+"{" and "}". No sizes or lengths are known ahead of time while, parsing, so the
+resulting Dart list created for the "data" key is append to as decoding happens.
+
+In contrast, decoding the standard message codec version of this message avoids
+utf8 decoding, instead operating on the bytes themselves. The only string constructed
+will be for the "data" key. The length of the list in the data field is encoded in
+the structure, meaning the correct length object can be allocated and filled in
+as decoding happens.
+
+### Schemaless
+
+Using standard message codec does not require a schema (like protobuf) or any
+generated code. This makes it easy to use for dynamic messages and simplifies
+the integration into existing codebases.
+
+The tradeoff for this ease of use is that it becomes the application's
+responsibility to verify the structure of messages sent/received. There is also
+no automatic backwards compatibility like protobuf.
+
+## Getting started
+
+standard_message_codec can be used to encode and decode messages in either Flutter
+or pure Dart applications.
+
+<?code-excerpt "readme_excerpts.dart (Encoding)"?>
+```dart
+void main() {
+  final ByteData? data = const StandardMessageCodec().encodeMessage(<Object, Object>{
+    'foo': true,
+    3: 'fizz',
+  });
+  print('The encoded message is $data');
+}
+```

packages/standard_message_codec/example/README.md

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+# example_standard_message_codec
+
+A sample app for demonstrating the StandardMessageCodec

packages/standard_message_codec/example/lib/readme_excerpts.dart

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+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file exists solely to host compiled excerpts for README.md, and is not
+// intended for use as an actual example application.
+
+// #docregion Encoding
+import 'dart:typed_data';
+import 'package:standard_message_codec/standard_message_codec.dart';
+// #enddocregion Encoding
+
+void main() {
+  final ByteData? data =
+      const StandardMessageCodec().encodeMessage(<Object, Object>{
+    'foo': true,
+    3: 'fizz',
+  });
+  print('The encoded message is $data');
+}

packages/standard_message_codec/example/pubspec.yaml

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+name: standard_message_codec_examples
+description: Example code for standard message codec usage
+version: 0.0.1
+publish_to: none
+
+environment:
+  sdk: ">=2.17.0 <3.0.0"
+
+dependencies:
+  standard_message_codec:
+    path: ../