iOS: In case Xcode is installed but the version is too old. Advise the user to update.

packages/flutter_tools/lib/src/ios/device_ios.dart

diff --git a/packages/flutter_tools/lib/src/ios/device_ios.dart b/packages/flutter_tools/lib/src/ios/device_ios.dart
index 83d4695..0225701 100644
--- a/packages/flutter_tools/lib/src/ios/device_ios.dart
+++ b/packages/flutter_tools/lib/src/ios/device_ios.dart
@@ -248,7 +248,7 @@
   IOSSimulator(String id, { this.name }) : super(id);
 
   static List<IOSSimulator> getAttachedDevices() {
-    if (!xcode.isInstalled)
+    if (!xcode.isInstalledAndMeetsVersionCheck)
       return <IOSSimulator>[];
 
     return SimControl.getConnectedDevices().map((SimDevice device) {

packages/flutter_tools/lib/src/ios/ios_workflow.dart

diff --git a/packages/flutter_tools/lib/src/ios/ios_workflow.dart b/packages/flutter_tools/lib/src/ios/ios_workflow.dart
index 5cc4f70..ebbfb54 100644
--- a/packages/flutter_tools/lib/src/ios/ios_workflow.dart
+++ b/packages/flutter_tools/lib/src/ios/ios_workflow.dart
@@ -7,6 +7,7 @@
 import '../base/process.dart';
 import '../doctor.dart';
 import '../globals.dart';
+import 'mac.dart';
 
 class IOSWorkflow extends Workflow {
   IOSWorkflow() : super('iOS');
@@ -14,11 +15,11 @@
   bool get appliesToHostPlatform => Platform.isMacOS;
 
   // We need xcode (+simctl) to list simulator devices, and idevice_id to list real devices.
-  bool get canListDevices => xcode.isInstalled;
+  bool get canListDevices => xcode.isInstalledAndMeetsVersionCheck;
 
   // We need xcode to launch simulator devices, and ideviceinstaller and ios-deploy
   // for real devices.
-  bool get canLaunchDevices => xcode.isInstalled;
+  bool get canLaunchDevices => xcode.isInstalledAndMeetsVersionCheck;
 
   ValidationResult validate() {
     Validator iosValidator = new Validator(
@@ -27,7 +28,23 @@
     );
 
     Function _xcodeExists = () {
-      return xcode.isInstalled ? ValidationType.installed : ValidationType.missing;
+      if (xcode.isInstalledAndMeetsVersionCheck) {
+        return ValidationType.installed;
+      }
+
+      if (xcode.isInstalled) {
+        return ValidationType.partial;
+      }
+
+      return ValidationType.missing;
+    };
+
+    Function _xcodeVersionSatisfacotry = () {
+      if (xcode.isInstalledAndMeetsVersionCheck) {
+        return ValidationType.installed;
+      }
+
+      return ValidationType.missing;
     };
 
     Function _brewExists = () {
@@ -44,11 +61,20 @@
       return hasIdeviceId ? ValidationType.installed : ValidationType.missing;
     };
 
-    iosValidator.addValidator(new Validator(
+    Validator xcodeValidator = new Validator(
       'XCode',
       description: 'enable development for iOS devices',
       resolution: 'Download at https://developer.apple.com/xcode/download/',
       validatorFunction: _xcodeExists
+    );
+
+    iosValidator.addValidator(xcodeValidator);
+
+    xcodeValidator.addValidator(new Validator(
+      'XCode',
+      description: 'Xcode version is at least $kXcodeRequiredVersionMajor.$kXcodeRequiredVersionMinor',
+      resolution: 'Download the latest version or update via the Mac App Store',
+      validatorFunction: _xcodeVersionSatisfacotry
     ));
 
     Validator brewValidator = new Validator(

packages/flutter_tools/lib/src/ios/mac.dart

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 47f5e79..d2aacc3 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -5,10 +5,49 @@
 import '../base/context.dart';
 import '../base/process.dart';
 
+const int kXcodeRequiredVersionMajor = 7;
+const int kXcodeRequiredVersionMinor = 2;
+
 class XCode {
   static void initGlobal() {
     context[XCode] = new XCode();
   }
 
-  bool get isInstalled => exitsHappy(<String>['xcode-select', '--print-path']);
+  bool get isInstalledAndMeetsVersionCheck => isInstalled && xcodeVersionSatisfactory;
+
+  bool _isInstalled;
+  bool get isInstalled {
+    if (_isInstalled != null) {
+      return _isInstalled;
+    }
+
+    _isInstalled = exitsHappy(<String>['xcode-select', '--print-path']);
+    return _isInstalled;
+  }
+
+  bool _xcodeVersionSatisfactory;
+  bool get xcodeVersionSatisfactory {
+    if (_xcodeVersionSatisfactory != null) {
+      return _xcodeVersionSatisfactory;
+    }
+
+    try {
+      String output = runSync(<String>['xcodebuild', '-version']);
+      RegExp regex = new RegExp(r'Xcode ([0-9.]+)');
+
+      String version = regex.firstMatch(output).group(1);
+      List<String> components = version.split('.');
+
+      int major = int.parse(components[0]);
+      int minor = components.length == 1 ? 0 : int.parse(components[1]);
+
+      _xcodeVersionSatisfactory = major >= kXcodeRequiredVersionMajor && minor >= kXcodeRequiredVersionMinor;
+      return _xcodeVersionSatisfactory;
+    } catch (error) {
+      _xcodeVersionSatisfactory = false;
+      return false;
+    }
+
+    return false;
+  }
 }