iOS: fail build wth error if Python six missing (#9135)

Xcode builds depend on the Python 'six' module. If not present, exit
immediately with a useful error message.

The six module is included in the system default Python installation. We
perform this check in case a custom Python install has higher priority
on $PATH; e.g., due to a Homebrew or MacPorts installation.

This extracts an existing doctor check to use it during the build step
as well.

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

@@ -41,7 +41,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
 
   bool get hasHomebrew => os.which('brew') != null;
 
-  bool get hasPythonSixModule => exitsHappy(<String>['python', '-c', 'import six']);
+  bool get hasPythonSixModule => kPythonSix.isInstalled;
 
   bool get hasCocoaPods => exitsHappy(<String>['pod', '--version']);
 
@@ -124,10 +124,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
       pythonStatus = ValidationType.installed;
     } else {
       pythonStatus = ValidationType.missing;
-      messages.add(new ValidationMessage.error(
-        'Python installation missing module "six".\n'
-        'Install via \'pip install six\' or \'sudo easy_install six\'.'
-      ));
+      messages.add(new ValidationMessage.error(kPythonSix.errorMessage));
     }
 
     // brew installed

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

@@ -27,6 +27,23 @@ import 'xcodeproj.dart';
 const int kXcodeRequiredVersionMajor = 7;
 const int kXcodeRequiredVersionMinor = 0;
 
+// The Python `six` module is a dependency for Xcode builds, and installed by
+// default, but may not be present in custom Python installs; e.g., via
+// Homebrew.
+const PythonModule kPythonSix = const PythonModule('six');
+
+class PythonModule {
+  const PythonModule(this.name);
+
+  final String name;
+
+  bool get isInstalled => exitsHappy(<String>['python', '-c', 'import $name']);
+
+  String get errorMessage =>
+    'Missing Xcode dependency: Python module "$name".\n'
+    'Install via \'pip install $name\' or \'sudo easy_install $name\'.';
+}
+
 class Xcode {
   Xcode() {
     _eulaSigned = false;
@@ -125,6 +142,11 @@ Future<XcodeBuildResult> buildXcodeProject({
   if (!_checkXcodeVersion())
     return new XcodeBuildResult(success: false);
 
+  if (!kPythonSix.isInstalled) {
+    printError(kPythonSix.errorMessage);
+    return new XcodeBuildResult(success: false);
+  }
+
   // Before the build, all service definitions must be updated and the dylibs
   // copied over to a location that is suitable for Xcodebuild to find them.
   final Directory appDirectory = fs.directory(app.appDirectory);