Deprecate assignment to non-existent global variables (#601)

See #600
See sass/sass#2606

CHANGELOG.md

@@ -1,5 +1,11 @@
 ## 1.17.2
 
+* Deprecate `!global` variable assignments to variables that aren't yet defined.
+  This deprecation message can be avoided by assigning variables to `null` at
+  the top level before globally assigning values to them.
+
+### Dart API
+
 * Explicitly mark classes that were never intended to be subclassed or
   implemented as "sealed".
 

lib/src/visitor/async_evaluate.dart

@@ -1124,6 +1124,16 @@ class _EvaluateVisitor
       if (value != null && value != sassNull) return null;
     }
 
+    if (node.isGlobal && !_environment.globalVariableExists(node.name)) {
+      _logger.warn(
+          "As of Dart Sass 2.0.0, !global assignments won't be able to\n"
+          "declare new variables. Consider adding `\$${node.name}: null` at "
+          "the top level.",
+          span: node.span,
+          trace: _stackTrace(node.span),
+          deprecation: true);
+    }
+
     _environment.setVariable(
         node.name,
         (await node.expression.accept(this)).withoutSlash(),

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 08c3aaa09f3be71dd315bf36665e249983ce3d53
+// Checksum: 607d30ac9d49b341367f71b69bed09f19f93e77d
 //
 // ignore_for_file: unused_import
 
@@ -1119,6 +1119,16 @@ class _EvaluateVisitor
       if (value != null && value != sassNull) return null;
     }
 
+    if (node.isGlobal && !_environment.globalVariableExists(node.name)) {
+      _logger.warn(
+          "As of Dart Sass 2.0.0, !global assignments won't be able to\n"
+          "declare new variables. Consider adding `\$${node.name}: null` at "
+          "the top level.",
+          span: node.span,
+          trace: _stackTrace(node.span),
+          deprecation: true);
+    }
+
     _environment.setVariable(
         node.name,
         node.expression.accept(this).withoutSlash(),