Extract Xxd class for iOS AOT snapshotting (#17006)

This fixes and re-lands 4bb7496b, which
was reverted in ceade39c due to test
failures caused by a bad rebase.

packages/flutter_tools/lib/src/commands/build_aot.dart

@@ -16,7 +16,7 @@ import '../build_info.dart';
 import '../compile.dart';
 import '../dart/package_map.dart';
 import '../globals.dart';
-import '../ios/mac.dart' show xcode;
+import '../ios/mac.dart' show xcode, xxd;
 import '../resident_runner.dart';
 import '../runner/flutter_command.dart';
 import 'build.dart';
@@ -422,12 +422,14 @@ Future<String> _buildAotSnapshot(
       await fs.file(vmSnapshotData).rename(fs.path.join(outputDir.path, kVmSnapshotData));
       await fs.file(isolateSnapshotData).rename(fs.path.join(outputDir.path, kIsolateSnapshotData));
 
-      await runCheckedAsync(<String>[
-        'xxd', '--include', kVmSnapshotData, fs.path.basename(kVmSnapshotDataC)
-      ], workingDirectory: outputDir.path);
-      await runCheckedAsync(<String>[
-        'xxd', '--include', kIsolateSnapshotData, fs.path.basename(kIsolateSnapshotDataC)
-      ], workingDirectory: outputDir.path);
+      await xxd.run(
+        <String>['--include', kVmSnapshotData, fs.path.basename(kVmSnapshotDataC)],
+        workingDirectory: outputDir.path,
+      );
+      await xxd.run(
+        <String>['--include', kIsolateSnapshotData, fs.path.basename(kIsolateSnapshotDataC)],
+        workingDirectory: outputDir.path,
+      );
 
       await xcode.cc(commonBuildOptions.toList()..addAll(<String>['-c', kVmSnapshotDataC, '-o', kVmSnapshotDataO]));
       await xcode.cc(commonBuildOptions.toList()..addAll(<String>['-c', kIsolateSnapshotDataC, '-o', kIsolateSnapshotDataO]));

packages/flutter_tools/lib/src/context_runner.dart

@@ -74,6 +74,7 @@ Future<T> runInContext<T>(
       Usage: () => new Usage(),
       Xcode: () => new Xcode(),
       XcodeProjectInterpreter: () => new XcodeProjectInterpreter(),
+      Xxd: () => new Xxd(),
     },
   );
 }

packages/flutter_tools/lib/src/ios/mac.dart

@@ -39,6 +39,8 @@ IMobileDevice get iMobileDevice => context[IMobileDevice];
 
 Xcode get xcode => context[Xcode];
 
+Xxd get xxd => context[Xxd];
+
 class PythonModule {
   const PythonModule(this.name);
 
@@ -103,6 +105,12 @@ class IMobileDevice {
   }
 }
 
+class Xxd {
+  Future<RunResult> run(List<String> args, {String workingDirectory}) {
+    return runCheckedAsync(<String>['xxd']..addAll(args), workingDirectory: workingDirectory);
+  }
+}
+
 class Xcode {
   bool get isInstalledAndMeetsVersionCheck => isInstalled && isVersionSatisfactory;