Minor type inference fixes to do with null and bottom types.

pkg/front_end/testcases/inference/block_bodied_lambdas_infer_bottom_sync_star.dart.strong.expect

Index: pkg/front_end/testcases/inference/block_bodied_lambdas_infer_bottom_sync_star.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/block_bodied_lambdas_infer_bottom_sync_star.dart.strong.expect b/pkg/front_end/testcases/inference/block_bodied_lambdas_infer_bottom_sync_star.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..8f9f12fc14cf35dddb0cd6006b6f186c3b71098f
--- /dev/null
+++ b/pkg/front_end/testcases/inference/block_bodied_lambdas_infer_bottom_sync_star.dart.strong.expect
@@ -0,0 +1,23 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  () → core::Iterable<core::Null> f = () → core::Iterable<core::Null> /* originally sync* */ {
+    dynamic :await_jump_var = 0;
+    dynamic :await_ctx_var;
+    function :sync_op(core::Iterator<dynamic> :iterator) → core::bool yielding {
+      {
+        {
+          :iterator._current = null;
+          [yield] true;
+        }
+      }
+      return false;
+    }
+    return new core::_SyncIterable::•(:sync_op);
+  };
+  core::Iterable<dynamic> y = f.call();
+  core::Iterable<core::String> z = f.call();
+  core::String s = f.call().first;
+}