Issue 53683. Give a not existing file different API signature, than no an empty file.

Bug: https://github.com/dart-lang/sdk/issues/53683
Change-Id: Ie4ac0eaea773e3a0d0da15ecb1d4645fe27a835c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/329660
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/analysis/file_state.dart

@@ -693,6 +693,7 @@ class FileState {
     return _fsState._logger.run('Create unlinked for $path', () {
       var unlinkedUnit = serializeAstUnlinked2(
         unit,
+        exists: exists,
         isDartCore: uriStr == 'dart:core',
       );
       var definedNames = computeDefinedNames(unit);
@@ -853,6 +854,7 @@ class FileState {
 
   static UnlinkedUnit serializeAstUnlinked2(
     CompilationUnit unit, {
+    required bool exists,
     required bool isDartCore,
   }) {
     UnlinkedLibraryDirective? libraryDirective;
@@ -984,8 +986,12 @@ class FileState {
       }
     }
 
+    final apiSignature = ApiSignature();
+    apiSignature.addBytes(computeUnlinkedApiSignature(unit));
+    apiSignature.addBool(exists);
+
     return UnlinkedUnit(
-      apiSignature: Uint8List.fromList(computeUnlinkedApiSignature(unit)),
+      apiSignature: apiSignature.toByteList(),
       augmentations: augmentations.toFixedList(),
       exports: exports.toFixedList(),
       imports: imports.toFixedList(),

pkg/analyzer/test/src/dart/analysis/driver_test.dart

@@ -1082,6 +1082,27 @@ var A = B;
     }, throwsArgumentError);
   }
 
+  test_changeFile_notExisting_toEmpty() async {
+    final b = newFile('/test/lib/b.dart', '''
+// ignore:unused_import
+import 'a.dart';
+''');
+
+    driver.addFile(b.path);
+    await waitForIdleWithoutExceptions();
+
+    // Has CompileTimeErrorCode.URI_DOES_NOT_EXIST
+    expect(allResults.withPath(b.path).errors, isNotEmpty);
+    allResults.clear();
+
+    final a = newFile('/test/lib/a.dart', '');
+    driver.changeFile(a.path);
+    await waitForIdleWithoutExceptions();
+
+    // No errors anymore.
+    expect(allResults.withPath(b.path).errors, isEmpty);
+  }
+
   test_changeFile_notUsed() async {
     var a = convertPath('/test/lib/a.dart');
     var b = convertPath('/other/b.dart');