Add test for missing initializer on final instance variable

Add a test where a class that has no generative constructors
declares an instance variable which is final and has no initializing
expression. With https://github.com/dart-lang/language/pull/1130 this
is an error, and that error is expected by the test. Also test that
it is not an error to have an uninitialized final variable which is
abstract or external.

Change-Id: I6c4a46393f46587448612bd91fb99fa65f89704c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157490
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>

tests/language/constructor/missing_initializer_test.dart

@@ -0,0 +1,49 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that it is an error for a class with no generative constructors to
+// have a final instance variable without an initializing expression, except
+// if it is `abstract` or `external`. The latter also holds in a class with
+// generative constructors.
+
+// Has factory, hence no default, hence no generative constructors.
+abstract class A {
+  final dynamic n;
+  //            ^
+  // [analyzer] COMPILE_TIME_ERROR.FINAL_NOT_INITIALIZED
+  // [cfe] unspecified
+
+  // Uninitialized, but no errors.
+  abstract final int x1;
+  abstract final int? x2;
+  external final String x3;
+  external final String? x4;
+
+  factory A() = B;
+}
+
+class B implements A {
+  dynamic get n => 1;
+  int get x1 => 1;
+  int? get x2 => null;
+  String get x3 => "";
+  String? get x4 => null;
+}
+
+class C = Object with A;
+
+// Has a generative constructor: default.
+abstract class D {
+  // Uninitialized, but no errors.
+  abstract final int x1;
+  abstract final int? x2;
+  external final String x3;
+  external final String? x4;
+}
+
+void main() {
+  A();
+  C();
+  var _ = D;
+}