Make Stream abstract as it is in the actual SDK.

pkg/analyzer/test/src/task/strong/inferred_type_test.dart

Index: pkg/analyzer/test/src/task/strong/inferred_type_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
index 03567041782c7dcead4d8f428cbf5cd9c8b58343..7a3c312e1673bc5d16b61bbf7c23148ab084c897 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -923,9 +923,14 @@ Future test() async {
   void test_downwardsInferenceForEach() {
     checkFile('''
 import 'dart:async';
+
+abstract class MyStream<T> extends Stream<T> {
+  factory MyStream() => null;
+}
+
 Future main() async {
   for(int x in /*info:INFERRED_TYPE_LITERAL*/[1, 2, 3]) {}
-  await for(int x in /*info:INFERRED_TYPE_ALLOCATION*/new Stream()) {}
+  await for(int x in /*info:INFERRED_TYPE_ALLOCATION*/new MyStream()) {}
 }
 ''');
   }
@@ -1564,11 +1569,16 @@ void main() {
   void test_downwardsInferenceYieldYieldStar() {
     checkFile('''
 import 'dart:async';
+
+abstract class MyStream<T> extends Stream<T> {
+  factory MyStream() => null;
+}
+
 Stream<List<int>> foo() async* {
   yield /*info:INFERRED_TYPE_LITERAL*/[];
-  yield /*error:YIELD_OF_INVALID_TYPE*/new Stream();
+  yield /*error:YIELD_OF_INVALID_TYPE*/new MyStream();
   yield* /*error:YIELD_OF_INVALID_TYPE*/[];
-  yield* /*info:INFERRED_TYPE_ALLOCATION*/new Stream();
+  yield* /*info:INFERRED_TYPE_ALLOCATION*/new MyStream();
 }
 
 Iterable<Map<int, int>> bar() sync* {