build: add stylelint rule to avoid top-level ampersands in themes (#10280)

* Adds a stylelint rule that will log a warning for top-level ampersand selectors inside of mixins. This will help us avoid issues like #9991 in the future.
* Makes the `mat-radio-color` internal.

tools/stylelint/no-top-level-ampersand-in-mixin/index.js

@@ -0,0 +1,44 @@
+const stylelint = require('stylelint');
+const path = require('path');
+const ruleName = 'material/no-top-level-ampersand-in-mixin';
+const messages = stylelint.utils.ruleMessages(ruleName, {
+  expected: () => `Selectors starting with an ampersand ` +
+                  `are not allowed inside top-level mixin rules`
+});
+
+/**
+ * Stylelint rule that doesn't allow for the top-level rules inside a
+ * mixin to start with an ampersand. Does not apply to internal mixins.
+ */
+const plugin = stylelint.createPlugin(ruleName, (isEnabled, options) => {
+  return (root, result) => {
+    if (!isEnabled) return;
+
+    const filePattern = new RegExp(options.filePattern);
+    const fileName = path.basename(root.source.input.file);
+
+    if (!filePattern.test(fileName)) return;
+
+    root.walkAtRules(node => {
+      // Skip non-mixin atrules and internal mixins.
+      if (node.name !== 'mixin' || node.params.indexOf('_') === 0) {
+        return;
+      }
+
+      node.nodes.forEach(childNode => {
+        if (childNode.type === 'rule' && childNode.selector.indexOf('&') === 0) {
+          stylelint.utils.report({
+            result,
+            ruleName,
+            message: messages.expected(),
+            node: childNode
+          });
+        }
+      });
+    });
+  };
+});
+
+plugin.ruleName = ruleName;
+plugin.messages = messages;
+module.exports = plugin;