fix analysis errors (#3677)

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

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 b7fe3a0..8d8bffc 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -207,7 +207,7 @@
 }
 
 Future<Null> _addServicesToBundle(Directory bundle) async {
-  List<Map<String, String>> services = [];
+  List<Map<String, String>> services = <Map<String, String>>[];
   printTrace("Trying to resolve native pub services.");
 
   // Step 1: Parse the service configuration yaml files present in the service
@@ -237,18 +237,18 @@
       continue;
     }
     // Shell out so permissions on the dylib are preserved.
-    runCheckedSync(['/bin/cp', dylib.path, frameworksDirectory.path]);
+    runCheckedSync(<String>['/bin/cp', dylib.path, frameworksDirectory.path]);
   }
 }
 
 void _copyServiceDefinitionsManifest(List<Map<String, String>> services, File manifest) {
   printTrace("Creating service definitions manifest at '${manifest.path}'");
-  List<Map<String, String>> jsonServices = services.map((Map<String, String> service) => {
+  List<Map<String, String>> jsonServices = services.map((Map<String, String> service) => <String, String>{
     'name': service['name'],
     // Since we have already moved it to the Frameworks directory. Strip away
     // the directory and basenames.
     'framework': path.basenameWithoutExtension(service['ios-framework'])
   }).toList();
-  Map<String, dynamic> json = { 'services' : jsonServices };
+  Map<String, dynamic> json = <String, dynamic>{ 'services' : jsonServices };
   manifest.writeAsStringSync(JSON.encode(json), mode: FileMode.WRITE, flush: true);
 }