enable lint prefer_single_quotes (#12665)

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 e5f75f8..644f66a 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -286,7 +286,7 @@
       commands.addAll(<String>[
         '-workspace', fs.path.basename(entity.path),
         '-scheme', scheme,
-        "BUILD_DIR=${fs.path.absolute(getIosBuildDirectory())}",
+        'BUILD_DIR=${fs.path.absolute(getIosBuildDirectory())}',
       ]);
       break;
     }
@@ -447,20 +447,20 @@
 
 Future<Null> _addServicesToBundle(Directory bundle) async {
   final List<Map<String, String>> services = <Map<String, String>>[];
-  printTrace("Trying to resolve native pub services.");
+  printTrace('Trying to resolve native pub services.');
 
   // Step 1: Parse the service configuration yaml files present in the service
   //         pub packages.
   await parseServiceConfigs(services);
-  printTrace("Found ${services.length} service definition(s).");
+  printTrace('Found ${services.length} service definition(s).');
 
   // Step 2: Copy framework dylibs to the correct spot for xcodebuild to pick up.
-  final Directory frameworksDirectory = fs.directory(fs.path.join(bundle.path, "Frameworks"));
+  final Directory frameworksDirectory = fs.directory(fs.path.join(bundle.path, 'Frameworks'));
   await _copyServiceFrameworks(services, frameworksDirectory);
 
   // Step 3: Copy the service definitions manifest at the correct spot for
   //         xcodebuild to pick up.
-  final File manifestFile = fs.file(fs.path.join(bundle.path, "ServiceDefinitions.json"));
+  final File manifestFile = fs.file(fs.path.join(bundle.path, 'ServiceDefinitions.json'));
   _copyServiceDefinitionsManifest(services, manifestFile);
 }
 
@@ -470,7 +470,7 @@
   for (Map<String, String> service in services) {
     final String dylibPath = await getServiceFromUrl(service['ios-framework'], service['root'], service['name']);
     final File dylib = fs.file(dylibPath);
-    printTrace("Copying ${dylib.path} into bundle.");
+    printTrace('Copying ${dylib.path} into bundle.');
     if (!dylib.existsSync()) {
       printError("The service dylib '${dylib.path}' does not exist.");
       continue;

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

diff --git a/packages/flutter_tools/lib/src/ios/plist_utils.dart b/packages/flutter_tools/lib/src/ios/plist_utils.dart
index 6d727d4..c0faca6 100644
--- a/packages/flutter_tools/lib/src/ios/plist_utils.dart
+++ b/packages/flutter_tools/lib/src/ios/plist_utils.dart
@@ -5,8 +5,8 @@
 import '../base/file_system.dart';
 import '../base/process.dart';
 
-const String kCFBundleIdentifierKey = "CFBundleIdentifier";
-const String kCFBundleShortVersionStringKey = "CFBundleShortVersionString";
+const String kCFBundleIdentifierKey = 'CFBundleIdentifier';
+const String kCFBundleShortVersionStringKey = 'CFBundleShortVersionString';
 
 String getValueFromFile(String plistFilePath, String key) {
   // TODO(chinmaygarde): For now, we only need to read from plist files on a mac