dart codegen improvements (#3440)

* fix: query params may be ints as well

don't assume query params are Strings or List<String>s

* fix dart: use correct type for QueryParams

type is Iterable, not List

only check params for != null if they haven't been checked already

add some casts to make strong mode happy

update petstore

* feat dart: enable strong mode

* feat dart: generate both browser and vm library

move petstore client into subdirectory and reference
generated libraries using path in pubspec.yaml

* feat dart: refactor petstore test.

The new layout resembles a real project.

petstore test is now strong mode compatible.

* feat: add DateTimerParser transformer

* feat: improved exception reporting

* feat[dart]: handle patch requests

* fix[dart]: typo

* fix dart: remove findPetsByTag test; enable strong-mode

find pets by tag call is deprecated and returns 500

generated library is now strong mode compatible

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/DartClientCodegen.java

@@ -140,6 +140,7 @@ public class DartClientCodegen extends DefaultCodegen implements CodegenConfig {
 
         final String libFolder = sourceFolder + File.separator + "lib";
         supportingFiles.add(new SupportingFile("pubspec.mustache", "", "pubspec.yaml"));
+        supportingFiles.add(new SupportingFile("analysis_options.mustache", "", ".analysis_options"));
         supportingFiles.add(new SupportingFile("api_client.mustache", libFolder, "api_client.dart"));
         supportingFiles.add(new SupportingFile("api_exception.mustache", libFolder, "api_exception.dart"));
         supportingFiles.add(new SupportingFile("api_helper.mustache", libFolder, "api_helper.dart"));