Add a --[no]-packages-dir flag.

lib/src/command/get.dart

Index: lib/src/command/get.dart
diff --git a/lib/src/command/get.dart b/lib/src/command/get.dart
index c86e904881d379f2ef08d7112debccf179af5652..ea2b900bfe67b86191729a5d7cc3d1876ee8bcd9 100644
--- a/lib/src/command/get.dart
+++ b/lib/src/command/get.dart
@@ -25,11 +25,16 @@ class GetCommand extends PubCommand {
 
     argParser.addFlag('precompile', defaultsTo: true,
         help: "Precompile executables and transformed dependencies.");
+
+    argParser.addFlag('packages-dir',
+        negatable: true, defaultsTo: true,
+        help: "Generate a packages/ directory when installing packages.");
   }
 
   Future run() {
     return entrypoint.acquireDependencies(SolveType.GET,
         dryRun: argResults['dry-run'],
-        precompile: argResults['precompile']);
+        precompile: argResults['precompile'],
+        packagesDir: argResults['packages-dir']);
   }
 }