Revert "Revert "Add built-in variables e and pi. (#907)""

This reverts commit 3c64add.

CHANGELOG.md

@@ -6,6 +6,8 @@
   * `hypot()`: given *n* numbers, outputs the length of the *n*-dimensional
     vector that has components equal to each of the inputs.
 
+* Add the variables `$pi` and `$e` to the built-in "sass:math" module.
+
 ## 1.24.4
 
 ### JavaScript API

lib/src/functions/math.dart

@@ -25,7 +25,10 @@ final global = UnmodifiableListView([
 final module = BuiltInModule("math", functions: [
   _abs, _ceil, _clamp, _compatible, _floor, _hypot, _isUnitless, _max, _min, //
   _percentage, _randomFunction, _round, _unit,
-]);
+], variables: {
+  "e": SassNumber(math.e),
+  "pi": SassNumber(math.pi),
+});
 
 /// Returns a [Callable] named [name] that transforms a number's value
 /// using [transform] and preserves its units.

lib/src/module/built_in.dart

@@ -17,19 +17,22 @@ class BuiltInModule<T extends AsyncCallable> implements Module<T> {
   final Uri url;
   final Map<String, T> functions;
   final Map<String, T> mixins;
+  final Map<String, Value> variables;
 
   List<Module<T>> get upstream => const [];
-  Map<String, Value> get variables => const {};
   Map<String, AstNode> get variableNodes => const {};
   Extender get extender => Extender.empty;
   CssStylesheet get css => CssStylesheet.empty(url: url);
   bool get transitivelyContainsCss => false;
   bool get transitivelyContainsExtensions => false;
 
-  BuiltInModule(String name, {Iterable<T> functions, Iterable<T> mixins})
+  BuiltInModule(String name,
+      {Iterable<T> functions, Iterable<T> mixins, Map<String, Value> variables})
       : url = Uri(scheme: "sass", path: name),
         functions = _callableMap(functions),
-        mixins = _callableMap(mixins);
+        mixins = _callableMap(mixins),
+        variables =
+            variables == null ? const {} : UnmodifiableMapView(variables);
 
   /// Returns a map from [callables]' names to their values.
   static Map<String, T> _callableMap<T extends AsyncCallable>(
@@ -40,7 +43,10 @@ class BuiltInModule<T extends AsyncCallable> implements Module<T> {
               {for (var callable in callables) callable.name: callable}));
 
   void setVariable(String name, Value value, AstNode nodeWithSpan) {
-    throw SassScriptException("Undefined variable.");
+    if (!variables.containsKey(name)) {
+      throw SassScriptException("Undefined variable.");
+    }
+    throw SassScriptException("Cannot modify built-in variable.");
   }
 
   Module<T> cloneCss() => this;