Add --all-platforms option to `flutter precache` (#5254)

packages/flutter_tools/lib/src/cache.dart

@@ -25,6 +25,10 @@ class Cache {
   // Initialized by FlutterCommandRunner on startup.
   static String flutterRoot;
 
+  // Whether to cache artifacts for all platforms. Defaults to only caching
+  // artifacts for the current platform.
+  bool includeAllPlatforms = false;
+
   static RandomAccessFile _lock;
   static bool _lockEnabled = true;
 
@@ -238,7 +242,9 @@ class FlutterEngine {
       'android-x86',
     ];
 
-    if (Platform.isMacOS)
+    if (cache.includeAllPlatforms)
+      dirs.addAll(<String>['ios', 'ios-profile', 'ios-release', 'linux-x64']);
+    else if (Platform.isMacOS)
       dirs.addAll(<String>['ios', 'ios-profile', 'ios-release']);
     else if (Platform.isLinux)
       dirs.add('linux-x64');

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

@@ -8,6 +8,11 @@ import '../globals.dart';
 import '../runner/flutter_command.dart';
 
 class PrecacheCommand extends FlutterCommand {
+  PrecacheCommand() {
+    argParser.addFlag('all-platforms', abbr: 'a', negatable: false,
+        help: 'Precache artifacts for all platforms.');
+  }
+
   @override
   final String name = 'precache';
 
@@ -19,6 +24,9 @@ class PrecacheCommand extends FlutterCommand {
 
   @override
   Future<int> runInProject() async {
+    if (argResults['all-platforms'])
+      cache.includeAllPlatforms = true;
+
     if (cache.isUpToDate())
       printStatus('Already up-to-date.');
     else