Revert "Require Xcode 9.0 or later for iOS builds (#13608)" (#13612)

Our devicelab machines have not yet been upgraded to Xcode 9. Once this
has been done, I'll re-land this.

This reverts commit 0fac5f01.

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

@@ -25,7 +25,7 @@ import 'cocoapods.dart';
 import 'code_signing.dart';
 import 'xcodeproj.dart';
 
-const int kXcodeRequiredVersionMajor = 9;
+const int kXcodeRequiredVersionMajor = 8;
 const int kXcodeRequiredVersionMinor = 0;
 
 // The Python `six` module is a dependency for Xcode builds, and installed by
@@ -544,4 +544,4 @@ Future<bool> upgradePbxProjWithFlutterAssets(String app) async {
   lines.forEach(buffer.writeln);
   await xcodeProjectFile.writeAsString(buffer.toString());
   return true;
-}
+}
\ No newline at end of file

packages/flutter_tools/test/ios/mac_test.dart

@@ -187,7 +187,7 @@ void main() {
 
     testUsingContext('xcodeVersionSatisfactory is false when version is less than minimum', () {
       when(mockProcessManager.runSync(<String>['/usr/bin/xcodebuild', '-version']))
-          .thenReturn(new ProcessResult(1, 0, 'Xcode 8.3.3\nBuild version 8E3004b', ''));
+          .thenReturn(new ProcessResult(1, 0, 'Xcode 7.0.1\nBuild version 7A1001', ''));
       expect(xcode.xcodeVersionSatisfactory, isFalse);
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -203,7 +203,7 @@ void main() {
 
     testUsingContext('xcodeVersionSatisfactory is true when version meets minimum', () {
       when(mockProcessManager.runSync(<String>['/usr/bin/xcodebuild', '-version']))
-          .thenReturn(new ProcessResult(1, 0, 'Xcode 9.0\nBuild version 9A235', ''));
+          .thenReturn(new ProcessResult(1, 0, 'Xcode 8.3.3\nBuild version 8E3004b', ''));
       expect(xcode.xcodeVersionSatisfactory, isTrue);
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -211,7 +211,7 @@ void main() {
 
     testUsingContext('xcodeVersionSatisfactory is true when version exceeds minimum', () {
       when(mockProcessManager.runSync(<String>['/usr/bin/xcodebuild', '-version']))
-          .thenReturn(new ProcessResult(1, 0, 'Xcode 10.0\nBuild version 10A123', ''));
+          .thenReturn(new ProcessResult(1, 0, 'Xcode 9.0\nBuild version 9M137d', ''));
       expect(xcode.xcodeVersionSatisfactory, isTrue);
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,