[ddc] Adding test for cross module private symbols as function default values.

Change-Id: I54e29a3883b2e444945574d3d5e03f741c1e8716
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436020
Reviewed-by: Nicholas Shahan <[email protected]>

Author: Markzipan

tests/modular/private_symbols/a.dart

@@ -0,0 +1,5 @@
+import 's.dart';
+
+Object foo_a({Object foo = s}) {
+  return foo;
+}

tests/modular/private_symbols/b.dart

@@ -0,0 +1,5 @@
+import 's.dart';
+
+Object foo_b({Object foo = s}) {
+  return foo;
+}

tests/modular/private_symbols/main.dart

@@ -0,0 +1,10 @@
+import 'package:expect/expect.dart';
+import 'a.dart';
+import 'b.dart';
+
+void main() {
+  var s1 = foo_a();
+  var s2 = foo_b();
+  Expect.identical(s1, s2);
+  Expect.equals(s1, s2);
+}

tests/modular/private_symbols/modules.yaml

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+dependencies:
+  main: [a, b, expect]
+  a: s
+  b: s

tests/modular/private_symbols/s.dart

@@ -0,0 +1 @@
+const s = #_symbol;