Require Xcode >= 8.0 for iOS development (#10932)

This allows us to take advantage of improved command-line tooling (e.g.,
improvements in device listing in Instruments). Now that the engine is
built with Xcode 8 and the framework is tested against Xcode 8, this
reduces the set of configurations we need to support to allow us to
focus on the supported ones: Xcode 8 and Xcode 9.

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

@@ -25,7 +25,7 @@ import 'code_signing.dart';
 import 'ios_workflow.dart';
 import 'xcodeproj.dart';
 
-const int kXcodeRequiredVersionMajor = 7;
+const int kXcodeRequiredVersionMajor = 8;
 const int kXcodeRequiredVersionMinor = 0;
 
 // The Python `six` module is a dependency for Xcode builds, and installed by
@@ -294,8 +294,9 @@ Future<Null> diagnoseXcodeBuildFailure(XcodeBuildResult result, BuildableIOSApp
   }
   if (result.xcodeBuildExecution != null &&
       result.xcodeBuildExecution.buildForPhysicalDevice &&
-      // Make sure the user has specified at least the DEVELOPMENT_TEAM (for automatic Xcode 8)
-      // signing or the PROVISIONING_PROFILE (for manual signing or Xcode 7).
+      // Make sure the user has specified one of:
+      // DEVELOPMENT_TEAM (automatic signing)
+      // PROVISIONING_PROFILE (manual signing)
       !(app.buildSettings?.containsKey('DEVELOPMENT_TEAM')) == true || app.buildSettings?.containsKey('PROVISIONING_PROFILE') == true) {
     printError(noDevelopmentTeamInstruction, emphasis: true);
     return;
@@ -355,7 +356,7 @@ class XcodeBuildExecution {
 }
 
 final RegExp _xcodeVersionRegExp = new RegExp(r'Xcode (\d+)\..*');
-final String _xcodeRequirement = 'Xcode 7.0 or greater is required to develop for iOS.';
+final String _xcodeRequirement = 'Xcode $kXcodeRequiredVersionMajor.$kXcodeRequiredVersionMinor or greater is required to develop for iOS.';
 
 bool _checkXcodeVersion() {
   if (!platform.isMacOS)
@@ -363,7 +364,7 @@ bool _checkXcodeVersion() {
   try {
     final String version = runCheckedSync(<String>['xcodebuild', '-version']);
     final Match match = _xcodeVersionRegExp.firstMatch(version);
-    if (int.parse(match[1]) < 7) {
+    if (int.parse(match[1]) < kXcodeRequiredVersionMajor) {
       printError('Found "${match[0]}". $_xcodeRequirement');
       return false;
     }