Don't crash when parsing +. (#1865)

Closes #1856

CHANGELOG.md

@@ -3,6 +3,9 @@
 * Remove sourcemap comments from Sass sources. The generated sourcemap comment
   for the compiled CSS output remains unaffected.
 
+* Produce a better error message for a number with a leading `+` or `-`, a
+  decimal point, but no digits.
+
 ## 1.57.1
 
 * No user-visible changes.

lib/src/parse/stylesheet.dart

@@ -2529,7 +2529,10 @@ abstract class StylesheetParser extends Parser {
     // Don't complain about a dot after a number unless the number starts with a
     // dot. We don't allow a plain ".", but we need to allow "1." so that
     // "1..." will work as a rest argument.
-    _tryDecimal(allowTrailingDot: scanner.position != start.position);
+    _tryDecimal(
+        allowTrailingDot: scanner.position != start.position &&
+            first != $plus &&
+            first != $minus);
     _tryExponent();
 
     // Use Dart's built-in double parsing so that we don't accumulate

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.57.2
+version: 1.57.2-dev
 description: A Sass implementation in Dart.
 homepage: https://github.com/sass/dart-sass