Check for potential null operand in Not expressions.

pkg/analyzer/lib/src/generated/resolver.dart

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index b5d6e1dff9f4204d4916ee9166274bd60a6c6528..2d5fac3269d04c5bcd01af8a8d70c3c9d7b6a13e 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -665,6 +665,8 @@ class BestPracticesVerifier extends RecursiveAstVisitor<Object> {
     }
     if (condition is BinaryExpression) {
       _checkForPossibleNullConditionInBinaryExpression(condition);
+    } else if (condition is PrefixExpression) {
+      _checkForPossibleNullConditionInPrefixExpression(condition);
     } else {
       _checkForPossibleNullConditionInSimpleExpression(condition);
     }
@@ -686,6 +688,17 @@ class BestPracticesVerifier extends RecursiveAstVisitor<Object> {
   }
 
   /**
+   * Produce a hint if the operand of the given prefix [condition] could
+   * have a value of `null`.
+   */
+  void _checkForPossibleNullConditionInPrefixExpression(
+      PrefixExpression condition) {
+    if (condition.operator?.type == TokenType.BANG) {
+      _checkForPossibleNullCondition(condition.operand);
+    }
+  }
+
+  /**
    * Produce a hint if the given [condition] could have a value of `null`.
    */
   void _checkForPossibleNullConditionInSimpleExpression(Expression condition) {