Add support for binary compression of dynamic patches by the flutter tool. (#27754)

This reduces the typical dynamic patch size by more than 10x.

Also see https://github.com/flutter/engine/pull/7777 for decompression support in the runtime.

packages/flutter_tools/lib/src/android/gradle.dart

@@ -10,6 +10,7 @@ import 'package:meta/meta.dart';
 import '../android/android_sdk.dart';
 import '../application_package.dart';
 import '../artifacts.dart';
+import '../base/bsdiff.dart';
 import '../base/common.dart';
 import '../base/file_system.dart';
 import '../base/logger.dart';
@@ -498,6 +499,7 @@ Future<void> _buildGradleProjectV2(
         throwToolExit('Error: Could not find baseline package ${baselineApkFile.path}.');
 
       printStatus('Found baseline package ${baselineApkFile.path}.');
+      printStatus('Creating dynamic patch...');
       final Archive newApk = ZipDecoder().decodeBytes(apkFile.readAsBytesSync());
       final Archive oldApk = ZipDecoder().decodeBytes(baselineApkFile.readAsBytesSync());
 
@@ -519,7 +521,14 @@ Future<void> _buildGradleProjectV2(
           throwToolExit("Error: Dynamic patching doesn't support changes to ${newFile.name}.");
 
         final String name = fs.path.relative(newFile.name, from: 'assets/');
-        update.addFile(ArchiveFile(name, newFile.content.length, newFile.content));
+        if (name.contains('_snapshot_')) {
+          final List<int> diff = bsdiff(oldFile.content, newFile.content);
+          final int ratio = 100 * diff.length ~/ newFile.content.length;
+          printStatus('Deflated $name by ${ratio == 0 ? 99 : 100 - ratio}%');
+          update.addFile(ArchiveFile(name + '.bzdiff40', diff.length, diff));
+        } else {
+          update.addFile(ArchiveFile(name, newFile.content.length, newFile.content));
+        }
       }
 
       File updateFile;
@@ -567,7 +576,8 @@ Future<void> _buildGradleProjectV2(
 
       updateFile.parent.createSync(recursive: true);
       updateFile.writeAsBytesSync(ZipEncoder().encode(update), flush: true);
-      printStatus('Created dynamic patch ${updateFile.path}.');
+      final String patchSize = getSizeAsMB(updateFile.lengthSync());
+      printStatus('Created dynamic patch ${updateFile.path} ($patchSize).');
     }
   } else {
     final File bundleFile = _findBundleFile(project, buildInfo);

packages/flutter_tools/test/base/bsdiff_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:typed_data';
+
+import 'package:flutter_tools/src/base/bsdiff.dart';
+
+import '../src/common.dart';
+
+void main() {
+  group('Main', () {
+    test('generates diff', () {
+      final Uint8List a = Uint8List.fromList('Hello'.runes.toList());
+      final Uint8List b = Uint8List.fromList('World'.runes.toList());
+      final Uint8List c = Uint8List.fromList(bsdiff(a, b));
+      expect(bspatch(a, c), equals(b));
+    });
+  });
+}