Create abstraction layer for dart:io's Process commands (#7100)

With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.

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

diff --git a/packages/flutter_tools/lib/src/ios/devices.dart b/packages/flutter_tools/lib/src/ios/devices.dart
index d2aa790..9e05e19 100644
--- a/packages/flutter_tools/lib/src/ios/devices.dart
+++ b/packages/flutter_tools/lib/src/ios/devices.dart
@@ -9,6 +9,7 @@
 import '../application_package.dart';
 import '../base/os.dart';
 import '../base/process.dart';
+import '../base/process_manager.dart';
 import '../build_info.dart';
 import '../device.dart';
 import '../doctor.dart';
@@ -514,7 +515,7 @@
     Process process = forwardedPort.context;
 
     if (process != null) {
-      Process.killPid(process.pid);
+      processManager.killPid(process.pid);
     } else {
       printError("Forwarded port did not have a valid process");
     }

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 f727f38..b7eead4 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -11,6 +11,7 @@
 import '../application_package.dart';
 import '../base/context.dart';
 import '../base/process.dart';
+import '../base/process_manager.dart';
 import '../build_info.dart';
 import '../flx.dart' as flx;
 import '../globals.dart';
@@ -39,7 +40,7 @@
       } else {
         try {
           printTrace('xcrun clang');
-          ProcessResult result = Process.runSync('/usr/bin/xcrun', <String>['clang']);
+          ProcessResult result = processManager.runSync('/usr/bin/xcrun', <String>['clang']);
 
           if (result.stdout != null && result.stdout.contains('license'))
             _eulaSigned = false;

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

diff --git a/packages/flutter_tools/lib/src/ios/simulators.dart b/packages/flutter_tools/lib/src/ios/simulators.dart
index d0bae71..3c30dca 100644
--- a/packages/flutter_tools/lib/src/ios/simulators.dart
+++ b/packages/flutter_tools/lib/src/ios/simulators.dart
@@ -13,6 +13,7 @@
 import '../base/common.dart';
 import '../base/context.dart';
 import '../base/process.dart';
+import '../base/process_manager.dart';
 import '../build_info.dart';
 import '../device.dart';
 import '../flx.dart' as flx;
@@ -190,7 +191,7 @@
 
     List<String> args = <String>['simctl', 'list', '--json', section.name];
     printTrace('$_xcrunPath ${args.join(' ')}');
-    ProcessResult results = Process.runSync(_xcrunPath, args);
+    ProcessResult results = processManager.runSync(_xcrunPath, args);
     if (results.exitCode != 0) {
       printError('Error executing simctl: ${results.exitCode}\n${results.stderr}');
       return <String, Map<String, dynamic>>{};