Remove unused --packages argument to gen_snapshot. (#28101)

This command line argument has already been removed in the main
Dart repository, so building flutter with the latest dart fails.

packages/flutter_tools/lib/src/base/build.dart

@@ -46,13 +46,11 @@ class GenSnapshot {
 
   Future<int> run({
     @required SnapshotType snapshotType,
-    @required String packagesPath,
     IOSArch iosArch,
     Iterable<String> additionalArgs = const <String>[],
   }) {
     final List<String> args = <String>[
       '--causal_async_stacks',
-      '--packages=$packagesPath',
     ]..addAll(additionalArgs);
 
     final String snapshotterPath = getSnapshotterPath(snapshotType);
@@ -193,7 +191,6 @@ class AOTSnapshotter {
     final SnapshotType snapshotType = SnapshotType(platform, buildMode);
     final int genSnapshotExitCode = await genSnapshot.run(
       snapshotType: snapshotType,
-      packagesPath: packageMap.packagesPath,
       additionalArgs: genSnapshotArgs,
       iosArch: iosArch,
     );
@@ -537,7 +534,6 @@ class JITSnapshotter {
     final SnapshotType snapshotType = SnapshotType(platform, buildMode);
     final int genSnapshotExitCode = await genSnapshot.run(
       snapshotType: snapshotType,
-      packagesPath: packagesPath,
       additionalArgs: genSnapshotArgs,
     );
     if (genSnapshotExitCode != 0) {