Play it safe with null tokens/nodes (#24910).

lib/src/linter.dart

Index: lib/src/linter.dart
diff --git a/lib/src/linter.dart b/lib/src/linter.dart
index f58b81dba1944b770e0e79591e4acb272148d76c..dad741589ae6ca8fd3476d2525862372a6ff8534 100644
--- a/lib/src/linter.dart
+++ b/lib/src/linter.dart
@@ -262,11 +262,15 @@ abstract class LintRule extends Linter implements Comparable<LintRule> {
   AstVisitor getVisitor() => null;
 
   void reportLint(AstNode node) {
-    reporter.reportErrorForNode(new _LintCode(name, description), node, []);
+    if (node != null) {
+      reporter.reportErrorForNode(new _LintCode(name, description), node, []);
+    }
   }
 
   void reportLintForToken(Token token) {
-    reporter.reportErrorForToken(new _LintCode(name, description), token, []);
+    if (token != null) {
+      reporter.reportErrorForToken(new _LintCode(name, description), token, []);
+    }
   }
 
   void reportPubLint(PSNode node) {