Do not check for an existing package map if a command is going to run "pub...

Do not check for an existing package map if a command is going to run "pub get" to update the package map (#3202)

Also remove a redundant check for the --pub option in the run command

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

@@ -10,7 +10,6 @@ import 'package:path/path.dart' as path;
 import '../application_package.dart';
 import '../base/common.dart';
 import '../build_configuration.dart';
-import '../dart/pub.dart';
 import '../device.dart';
 import '../globals.dart';
 import '../runner/flutter_command.dart';
@@ -82,16 +81,6 @@ class RunCommand extends RunCommandBase {
   @override
   bool get requiresDevice => true;
 
-  @override
-  Future<int> run() async {
-    if (argResults['pub']) {
-      int exitCode = await pubGet();
-      if (exitCode != 0)
-        return exitCode;
-    }
-    return await super.run();
-  }
-
   @override
   Future<int> runInProject() async {
     bool clearLogs = argResults['clear-logs'];

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -154,11 +154,14 @@ abstract class FlutterCommand extends Command {
       }
     }
 
+    // Validate the current package map only if we will not be running "pub get" later.
+    if (!(_usesPubOption && argResults['pub'])) {
       String error = PackageMap.instance.checkValid();
       if (error != null) {
         printError(error);
         return false;
       }
+    }
 
     return true;
   }