converge flutter run and flutter drive onto one behavior (#7688)

...for the port arguments

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

@@ -8,7 +8,6 @@ import 'platform.dart';
 
 const int kDefaultObservatoryPort = 8100;
 const int kDefaultDiagnosticPort  = 8101;
-const int kDefaultDrivePort       = 8183;
 
 /// Return the absolute path of the user's home directory
 String get homeDirPath {

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

@@ -84,9 +84,6 @@ class DriveCommand extends RunCommandBase {
   // ignore: cancel_subscriptions
   StreamSubscription<String> _deviceLogSubscription;
 
-  int get observatoryPort => int.parse(argResults['observatory-port']);
-  int get diagnosticPort => int.parse(argResults['diagnostic-port']);
-
   @override
   Future<Null> verifyThenRunCommand() async {
     commandValidator();

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

@@ -189,24 +189,6 @@ class RunCommand extends RunCommandBase {
       return null;
     }
 
-    int observatoryPort;
-    if (argResults['observatory-port'] != null) {
-      try {
-        observatoryPort = int.parse(argResults['observatory-port']);
-      } catch (error) {
-        throwToolExit('Invalid port for `--observatory-port`: $error');
-      }
-    }
-
-    int diagnosticPort;
-    if (argResults['diagnostic-port'] != null) {
-      try {
-        diagnosticPort = int.parse(argResults['diagnostic-port']);
-      } catch (error) {
-        throwToolExit('Invalid port for `--diagnostic-port`: $error');
-      }
-    }
-
     if (device.isLocalEmulator && !isEmulatorBuildMode(getBuildMode()))
       throwToolExit('${toTitleCase(getModeName(getBuildMode()))} mode is not supported for emulators.');
 

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

@@ -76,6 +76,28 @@ abstract class FlutterCommand extends Command<Null> {
     );
   }
 
+  int get observatoryPort {
+    if (argResults['observatory-port'] != null) {
+      try {
+        return int.parse(argResults['observatory-port']);
+      } catch (error) {
+        throwToolExit('Invalid port for `--observatory-port`: $error');
+      }
+    }
+    return null;
+  }
+
+  int get diagnosticPort {
+    if (argResults['diagnostic-port'] != null) {
+      try {
+        return int.parse(argResults['diagnostic-port']);
+      } catch (error) {
+        throwToolExit('Invalid port for `--diagnostic-port`: $error');
+      }
+    }
+    return null;
+  }
+
   void addBuildModeFlags({ bool defaultToRelease: true }) {
     defaultBuildMode = defaultToRelease ? BuildMode.release : BuildMode.debug;