[analyzer] Improve dead code highlighting for null-aware expressions

Previously, when a null-aware property access, method invocation, or
index invocation was performed on a variable that was definitely
unassigned, the entire expression was highlighted as dead code.

This change modifies the `DeadCodeVerifier` to highlight only from the
null-aware operator (`?.`, `?[]`, etc.) to the end of the expression.
This provides a more precise and helpful diagnostic by highlighting
the exact portion of the code that can be safely removed.

Fixes #61251.

Bug: #61251
Change-Id: I4792b84d16a797f70b752a20e6affa69f3fe3d8c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444040
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/error/dead_code_verifier.dart

@@ -472,7 +472,8 @@ class NullSafetyDeadCodeVerifier {
     var flowAnalysis = _flowAnalysis;
     if (flowAnalysis == null) return;
 
-    var operatorType = operator?.type;
+    if (operator == null) return;
+    var operatorType = operator.type;
     if (operatorType != TokenType.QUESTION &&
         operatorType != TokenType.QUESTION_PERIOD &&
         operatorType != TokenType.QUESTION_PERIOD_PERIOD) {
@@ -494,7 +495,11 @@ class NullSafetyDeadCodeVerifier {
           node = parent!;
           parent = node.parent;
         }
-        _diagnosticReporter.atNode(node, WarningCode.DEAD_CODE);
+        _diagnosticReporter.atOffset(
+          offset: operator.offset,
+          length: node.end - operator.offset,
+          diagnosticCode: WarningCode.DEAD_CODE,
+        );
       }
     }
   }

pkg/analyzer/test/src/diagnostics/dead_code_test.dart

@@ -2234,7 +2234,7 @@ void f() {
   l?..[0]..length;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 29, 15)],
+      [error(WarningCode.DEAD_CODE, 30, 14)],
     );
   }
 
@@ -2246,7 +2246,7 @@ void f() {
   i?..toInt()..isEven;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 19)],
+      [error(WarningCode.DEAD_CODE, 24, 18)],
     );
   }
 
@@ -2258,7 +2258,7 @@ void f() {
   i?..sign..isEven;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 16)],
+      [error(WarningCode.DEAD_CODE, 24, 15)],
     );
   }
 
@@ -2270,7 +2270,7 @@ void f() {
   l?[0];
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 29, 5)],
+      [error(WarningCode.DEAD_CODE, 30, 4)],
     );
   }
 
@@ -2282,7 +2282,7 @@ void f() {
   l?[0][0];
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 35, 8)],
+      [error(WarningCode.DEAD_CODE, 36, 7)],
     );
   }
 
@@ -2294,7 +2294,7 @@ void f() {
   i?.truncate();
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 13)],
+      [error(WarningCode.DEAD_CODE, 24, 12)],
     );
   }
 
@@ -2306,7 +2306,7 @@ void f() {
   i?.truncate().truncate();
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 24)],
+      [error(WarningCode.DEAD_CODE, 24, 23)],
     );
   }
 
@@ -2318,7 +2318,7 @@ void f() {
   i?.truncate().sign;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 18)],
+      [error(WarningCode.DEAD_CODE, 24, 17)],
     );
   }
 
@@ -2330,7 +2330,7 @@ void f() {
   (i)?.sign;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 9)],
+      [error(WarningCode.DEAD_CODE, 26, 6)],
     );
   }
 
@@ -2342,7 +2342,7 @@ void f() {
   (i)?.sign.sign;
 }
 ''',
-      [error(WarningCode.DEAD_CODE, 23, 14)],
+      [error(WarningCode.DEAD_CODE, 26, 11)],
     );
   }