Forbid multiple !global or !default flags on the same variable

Closes #604

Author: nex3

CHANGELOG.md

@@ -7,6 +7,9 @@
   parsing associated with it. It is now parsed like any other unknown plain CSS
   at-rule, where Sass features are only allowed within `#{}` interpolation.
 
+* **Breaking change:** A variable definition may now only have a single
+  `!global` flag or `!default` flag, or one of each.
+
 ### Bogus Combinators
 
 * **Breaking change:** Selectors with more than one combinator in a row, such as

lib/src/parse/stylesheet.dart

@@ -255,13 +255,8 @@ abstract class StylesheetParser extends Parser {
       switch (identifier()) {
         case 'default':
           if (guarded) {
-            warnings.add((
-              deprecation: Deprecation.duplicateVarFlags,
-              message:
-                  '!default should only be written once for each variable.\n'
-                  'This will be an error in Dart Sass 2.0.0.',
-              span: scanner.spanFrom(flagStart),
-            ));
+            error('Only one !default is allowed per declaration.',
+                scanner.spanFrom(flagStart));
           }
           guarded = true;
 
@@ -272,13 +267,8 @@ abstract class StylesheetParser extends Parser {
               scanner.spanFrom(flagStart),
             );
           } else if (global) {
-            warnings.add((
-              deprecation: Deprecation.duplicateVarFlags,
-              message:
-                  '!global should only be written once for each variable.\n'
-                  'This will be an error in Dart Sass 2.0.0.',
-              span: scanner.spanFrom(flagStart),
-            ));
+            error('Only one !global is allowed per declaration.',
+                scanner.spanFrom(flagStart));
           }
           global = true;