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 32000e99686c3ced31ac7b2e5660275db7477a37..386c8c10b87165e0095de1c4b6fcc08d7cd672d5 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -3616,6 +3616,15 @@ test1() {
 ''');
   }
 
+  void test_nullCoalescingOperator() {
+    // Regression test for https://github.com/dart-lang/sdk/issues/26552

[Leaf, 2016/08/04 22:26:44]

Can you put a case in here to test for when we do have a context type to make
sure that doesn't regress in the future?  Unless there's already a separate test
for that.

[Jennifer Messerly, 2016/08/04 22:57:49]

Done.

+    checkFile(r'''
+List<int> x;
+var y = x ?? /*info:INFERRED_TYPE_LITERAL*/[];
+List<int> z = y;
+    ''');
+  }
+
   void test_nullLiteralShouldNotInferAsBottom() {
     // Regression test for https://github.com/dart-lang/dev_compiler/issues/47
     checkFile(r'''