Fix RemoveUnusedField when some SourceRange(s) being removed are fully covered.

Change-Id: I7ae8b9286c473a368def9c9db22b2f2b828c7b81
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214260
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/lib/src/services/correction/dart/remove_unused.dart

@@ -116,8 +116,9 @@ class RemoveUnusedField extends _RemoveUnused {
       sourceRanges.add(sourceRange);
     }
 
+    final uniqueSourceRanges = _uniqueSourceRanges(sourceRanges);
     await builder.addDartFileEdit(file, (builder) {
-      for (var sourceRange in sourceRanges) {
+      for (var sourceRange in uniqueSourceRanges) {
         builder.addDeletion(sourceRange);
       }
     });
@@ -182,6 +183,24 @@ class RemoveUnusedField extends _RemoveUnused {
     }
   }
 
+  /// Return [SourceRange]s that are not covered by other in [ranges].
+  /// If there is any intersection, it must be fully covered, never partially.
+  List<SourceRange> _uniqueSourceRanges(List<SourceRange> ranges) {
+    var result = <SourceRange>[];
+    candidates:
+    for (var candidate in ranges) {
+      for (var other in ranges) {
+        if (identical(candidate, other)) {
+          continue;
+        } else if (candidate.coveredBy(other)) {
+          continue candidates;
+        }
+      }
+      result.add(candidate);
+    }
+    return result;
+  }
+
   /// Return an instance of this class. Used as a tear-off in `FixProcessor`.
   static RemoveUnusedField newInstance() => RemoveUnusedField();
 }

pkg/analysis_server/test/src/services/correction/fix/remove_unused_field_test.dart

@@ -192,6 +192,23 @@ class A {
 ''');
   }
 
+  Future<void> test_unusedField_notUsed_assign_nested() async {
+    await resolveTestCode(r'''
+class A {
+  var _f;
+  main() {
+    _f = () { _f = 0; };
+  }
+}
+''');
+    await assertHasFix(r'''
+class A {
+  main() {
+  }
+}
+''');
+  }
+
   Future<void> test_unusedField_notUsed_compoundAssign() async {
     await resolveTestCode(r'''
 class A {