_downloadArtifacts (Web SDK) uses content-aware hashing in post-submit (flutter#174236)

Towards flutter#174225.

Will need to get cherry-picked into 3.35 and 3.36.

Author: matanlurey

engine/src/flutter/lib/web_ui/dev/common.dart

@@ -258,6 +258,31 @@ final String gitRevision = () {
   return (result.stdout as String).trim();
 }();
 
+final String contentHash = () {
+  final String executable;
+  final List<String> args;
+  if (io.Platform.isWindows) {
+    executable = 'powershell';
+    args = <String>[path.join('bin', 'internal', 'content_aware_hash.ps1')];
+  } else {
+    executable = path.join('bin', 'internal', 'content_aware_hash.sh');
+    args = <String>[];
+  }
+  final result = io.Process.runSync(
+    executable,
+    args,
+    workingDirectory: environment.flutterRootDir.path,
+    stderrEncoding: utf8,
+    stdoutEncoding: utf8,
+  );
+  if (result.exitCode != 0) {
+    throw ToolExit(
+      'Failed to get content hash. Exit code: ${result.exitCode} Error: ${result.stderr}',
+    );
+  }
+  return (result.stdout as String).trim();
+}();
+
 const String kChrome = 'chrome';
 const String kEdge = 'edge';
 const String kFirefox = 'firefox';

engine/src/flutter/lib/web_ui/dev/environment.dart

@@ -24,6 +24,7 @@ class Environment {
     final io.File self = io.File.fromUri(io.Platform.script);
 
     final io.Directory engineSrcDir = self.parent.parent.parent.parent.parent;
+    final io.Directory flutterRootDir = engineSrcDir.parent.parent;
     final io.Directory engineToolsDir = io.Directory(
       pathlib.join(engineSrcDir.path, 'flutter', 'tools'),
     );
@@ -52,6 +53,7 @@ class Environment {
       isMacosArm: isMacosArm,
       webUiRootDir: webUiRootDir,
       engineSrcDir: engineSrcDir,
+      flutterRootDir: flutterRootDir,
       engineToolsDir: engineToolsDir,
       outDir: outDir,
       wasmReleaseOutDir: wasmReleaseOutDir,
@@ -67,6 +69,7 @@ class Environment {
     required this.isMacosArm,
     required this.webUiRootDir,
     required this.engineSrcDir,
+    required this.flutterRootDir,
     required this.engineToolsDir,
     required this.outDir,
     required this.wasmReleaseOutDir,
@@ -137,6 +140,9 @@ class Environment {
     'dart2wasm.dart',
   );
 
+  /// Path to the root flutter directory (which itself contains `engine/src/flutter`).
+  final io.Directory flutterRootDir;
+
   /// Path to where github.com/flutter/engine is checked out inside the engine workspace.
   io.Directory get flutterDirectory => io.Directory(pathlib.join(engineSrcDir.path, 'flutter'));
   io.Directory get webSdkRootDir => io.Directory(pathlib.join(flutterDirectory.path, 'web_sdk'));

engine/src/flutter/lib/web_ui/dev/steps/copy_artifacts_step.dart

@@ -57,7 +57,7 @@ class CopyArtifactsStep implements PipelineStep {
     };
     final Uri url = Uri.https(
       'storage.googleapis.com',
-      '${realmComponent}flutter_infra_release/flutter/$gitRevision/flutter-web-sdk.zip',
+      '${realmComponent}flutter_infra_release/flutter/${realm == LuciRealm.Try ? gitRevision : contentHash}/flutter-web-sdk.zip',
     );
     final http.Response response = await http.Client().get(url);
     if (response.statusCode != 200) {