Enable and fix lint non_constant_identifier_names (#311)

Author: natebosch

analysis_options.yaml

@@ -36,7 +36,7 @@ linter:
   - list_remove_unrelated_type
   - no_adjacent_strings_in_list
   - no_duplicate_case_values
-  # non_constant_identifier_names
+  - non_constant_identifier_names
   - null_closures
   - omit_local_variable_types
   - only_throw_errors

lib/src/utils.dart

@@ -55,11 +55,11 @@ Encoding requiredEncodingForCharset(String charset) =>
 
 /// A regular expression that matches strings that are composed entirely of
 /// ASCII-compatible characters.
-final RegExp _ASCII_ONLY = RegExp(r'^[\x00-\x7F]+$');
+final _asciiOnly = RegExp(r'^[\x00-\x7F]+$');
 
 /// Returns whether [string] is composed entirely of ASCII-compatible
 /// characters.
-bool isPlainAscii(String string) => _ASCII_ONLY.hasMatch(string);
+bool isPlainAscii(String string) => _asciiOnly.hasMatch(string);
 
 /// Converts [input] into a [Uint8List].
 ///