Closes #12 & #13

[zmuranaka]

• Adds analysis_options.yaml using the flutter_lints package and address the warnings
• Give ImageCompressionFlutterWeb.handleMethodCall a return type
• Closes ImageCompressionFlutter.dummyCallNativeCode shouldn't be able to return null #13 by making it unable to return null
• Closes Validator.checkFileNameAndFormat incorrectly assert message #12 by fixing the assert message
• Update pubspecs