fix #26552, improve null coalescing inference

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 fe2bdfb5b3bf259875a65f89eba2c612f3d953e7..e17448aaadf72e35cded2f56eb7584d5005ea35d 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -3626,6 +3626,23 @@ test1() {
 ''');
   }
 
+  void test_nullCoalescingOperator() {
+    // Regression test for https://github.com/dart-lang/sdk/issues/26552
+    checkFile(r'''
+List<int> x;
+var y = x ?? /*info:INFERRED_TYPE_LITERAL*/[];
+List<int> z = y;
+    ''');
+    // Don't do anything if we already have a context type.
+    var unit = checkFile(r'''
+List<int> x;
+List<num> y = x ?? /*info:INFERRED_TYPE_LITERAL*/[];
+    ''');
+
+    expect(unit.topLevelVariables[1].initializer.returnType.toString(),
+        'List<num>');
+  }
+
   void test_nullLiteralShouldNotInferAsBottom() {
     // Regression test for https://github.com/dart-lang/dev_compiler/issues/47
     checkFile(r'''