Support converting a declaring param to an initializing param

Server already supports the opposite, so this just makes it possible to
go both ways.

This doesn't support converting a declaring function typed parameter.
We could add that, but I'm hoping it's rare enough to not be worth doing.

Other than that, I think I've covered all the cases, but please pay
special attention to the test coverage.

Change-Id: I400204ca820d0e0f1cc44075dd54946db79b2115
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506740
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -7,6 +7,7 @@ import 'package:analysis_server/src/services/correction/fix.dart';
 import 'package:analysis_server_plugin/edit/dart/correction_producer.dart';
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/ast/ast.dart';
 import 'package:analyzer_plugin/utilities/assist/assist.dart';
@@ -336,6 +337,32 @@ class ConvertToInitializingFormal extends ResolvedCorrectionProducer {
         field,
         assignment: statement,
       );
+    } else if (parameter is RegularFormalParameter &&
+        parameterElement is FieldFormalParameterElement) {
+      // This is a declaring parameter in a primary constructor.
+      var keyword = parameter.constFinalOrVarKeyword;
+      if (keyword == null) return;
+      var parameterName = parameter.name;
+      if (parameterName == null) return;
+      var container = node.thisOrAncestorOfType<CompilationUnitMember>();
+      if (container == null) return;
+      if (parameter.functionTypedSuffix != null) return;
+      await builder.addDartFileEdit(file, (builder) {
+        var type = parameter.type;
+        // Remove the `var` or `final` keyword, and add `this.`.
+        builder.addSimpleReplacement(
+          range.startStart(keyword, parameterName),
+          'this.',
+        );
+        // Add the field.
+        builder.insertField(container, (builder) {
+          builder.writeFieldDeclaration(
+            parameterName.lexeme,
+            isFinal: keyword.keyword == Keyword.FINAL,
+            type: type?.type,
+          );
+        });
+      });
     }
   }
 

pkg/analysis_server/test/src/services/correction/assist/convert_to_initializing_formal_test.dart

@@ -279,7 +279,37 @@ class A {
 ''');
   }
 
-  Future<void> test_parameterDeclaration_primary() async {
+  Future<void> test_parameterDeclaration_primary_declaring_final() async {
+    await resolveTestCode('''
+class C(final int ^test);
+''');
+    await assertHasAssist('''
+class C(this.test) {
+  final int test;
+}
+''');
+  }
+
+  Future<void>
+  test_parameterDeclaration_primary_declaring_functionTyped() async {
+    await resolveTestCode('''
+class C(var int ^test(String));
+''');
+    await assertNoAssist();
+  }
+
+  Future<void> test_parameterDeclaration_primary_declaring_var() async {
+    await resolveTestCode('''
+class C(var int ^test);
+''');
+    await assertHasAssist('''
+class C(this.test) {
+  int test;
+}
+''');
+  }
+
+  Future<void> test_parameterDeclaration_primary_regular() async {
     await resolveTestCode('''
 class C(int ^test) {
   int test;