[testing] Fix failure in test runner's batch compilation while deflaking tests.

All subclasses of Command must override indexedCopy.
A @subClassMustOverride annotation could have caught this problem.

Bug: https://github.com/dart-lang/sdk/issues/30175
Change-Id: I836cf58ae2233e3b2a5c7a494461e570aad5ffb9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250300
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

pkg/test_runner/lib/src/command.dart

@@ -266,6 +266,18 @@ class Dart2jsCompilationCommand extends CompilationCommand {
             workingDirectory: workingDirectory,
             index: index);
 
+  @override
+  Dart2jsCompilationCommand indexedCopy(int index) => Dart2jsCompilationCommand(
+      outputFile,
+      _bootstrapDependencies,
+      executable,
+      arguments,
+      environmentOverrides,
+      useSdk: useSdk,
+      alwaysCompile: _alwaysCompile,
+      workingDirectory: workingDirectory,
+      index: index);
+
   @override
   CommandOutput createOutput(int exitCode, bool timedOut, List<int> stdout,
       List<int> stderr, Duration time, bool compilationSkipped,
@@ -321,6 +333,15 @@ class DevCompilerCompilationCommand extends CompilationCommand {
             workingDirectory: workingDirectory,
             index: index);
 
+  @override
+  DevCompilerCompilationCommand indexedCopy(int index) =>
+      DevCompilerCompilationCommand(outputFile, _bootstrapDependencies,
+          executable, arguments, environmentOverrides,
+          compilerPath: compilerPath,
+          alwaysCompile: _alwaysCompile,
+          workingDirectory: workingDirectory,
+          index: index);
+
   @override
   CommandOutput createOutput(int exitCode, bool timedOut, List<int> stdout,
       List<int> stderr, Duration time, bool compilationSkipped,