Make ?? a compile-time constant operator.

tests/language/const_qq_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Check that ?? is compile-time constant.
+
+import "package:expect/expect.dart";
+
+const theNull = null;
+const notNull = const Object();
+
+// Top-level const field initializer.
+const test1 = theNull ?? notNull;
+const test2 = notNull ?? theNull;
+const test3 = theNull ?? theNull ?? notNull;
+const test4 = theNull ?? theNull ?? theNull;
+
+class P {
+  final v;
+  const P(this.v);
+}
+
+// Annotation parameter (not checked by test!)
+@P(theNull ?? notNull)
+@P(notNull ?? theNull)
+@P(theNull ?? theNull ?? notNull)
+@P(theNull ?? theNull ?? theNull)
+class C {
+  // Static const field initializer.
+  static const test5 = theNull ?? notNull;
+  static const test6 = notNull ?? theNull;
+  static const test7 = theNull ?? theNull ?? notNull;
+  static const test8 = theNull ?? theNull ?? theNull;
+
+  // (Constructor) parameter defaults.
+  final test9;
+  final test10;
+  final test11;
+  final test12;
+
+  // Const constructor initializer list.
+  final test13;
+  final test14;
+  final test15;
+  final test16;
+  final test17;
+
+  const C(x,
+          [this.test9  = theNull ?? notNull,
+           this.test10 = notNull ?? theNull,
+           this.test11 = theNull ?? theNull ?? notNull,
+           this.test12 = theNull ?? theNull ?? theNull])
+      : test13 = theNull ?? x,
+        test14 = notNull ?? x,
+        test15 = x ?? notNull,
+        test16 = theNull ?? theNull ?? x,
+        test17 = theNull ?? x ?? notNull;
+
+  List methodLocal() {
+    // Method local const variable initializer.
+    const test18 = theNull ?? notNull;
+    const test19 = notNull ?? theNull;
+    const test20 = theNull ?? theNull ?? notNull;
+    const test21 = theNull ?? theNull ?? theNull;
+
+    return const [test18, test19, test20, test21];
+  }
+
+  List expressionLocal() {
+    // Constant expression sub-expression.
+    return const [theNull ?? notNull,
+                  notNull ?? theNull,
+                  theNull ?? theNull ?? notNull,
+                  theNull ?? theNull ?? theNull];
+  }
+}
+
+main() {
+  Expect.identical(notNull, test1, "test1");
+  Expect.identical(notNull, test2, "test2");
+  Expect.identical(notNull, test3, "test3");
+  Expect.identical(theNull, test4, "test4");
+
+  Expect.identical(notNull, C.test5, "test5");
+  Expect.identical(notNull, C.test6, "test6");
+  Expect.identical(notNull, C.test7, "test7");
+  Expect.identical(theNull, C.test8, "test8");
+
+  const c1 = const C(null);
+  Expect.identical(notNull, c1.test9, "test9");
+  Expect.identical(notNull, c1.test10, "test10");
+  Expect.identical(notNull, c1.test11, "test11");
+  Expect.identical(theNull, c1.test12, "test12");
+
+  Expect.identical(theNull, c1.test13, "test13");
+  Expect.identical(notNull, c1.test14, "test14");
+  Expect.identical(notNull, c1.test15, "test15");
+  Expect.identical(theNull, c1.test16, "test16");
+  Expect.identical(notNull, c1.test17, "test17");
+
+  var list = c1.methodLocal();
+  Expect.identical(notNull, list[0], "test18");
+  Expect.identical(notNull, list[1], "test19");
+  Expect.identical(notNull, list[2], "test20");
+  Expect.identical(theNull, list[3], "test21");
+
+  list = c1.expressionLocal();
+  Expect.identical(notNull, list[0], "test22");
+  Expect.identical(notNull, list[1], "test23");
+  Expect.identical(notNull, list[2], "test24");
+  Expect.identical(theNull, list[3], "test25");
+
+  const c2 = const C(42);
+  Expect.identical(notNull, c2.test9, "test26");
+  Expect.identical(notNull, c2.test10, "test27");
+  Expect.identical(notNull, c2.test11, "test28");
+  Expect.identical(theNull, c2.test12, "test29");
+
+  Expect.identical(42     , c2.test13, "test30");
+  Expect.identical(notNull, c2.test14, "test31");
+  Expect.identical(42     , c2.test15, "test32");
+  Expect.identical(42     , c2.test16, "test33");
+  Expect.identical(42     , c2.test17, "test34");
+
+  list = c2.methodLocal();
+  Expect.identical(notNull, list[0], "test35");
+  Expect.identical(notNull, list[1], "test36");
+  Expect.identical(notNull, list[2], "test37");
+  Expect.identical(theNull, list[3], "test38");
+
+  list = c2.expressionLocal();
+  Expect.identical(notNull, list[0], "test39");
+  Expect.identical(notNull, list[1], "test40");
+  Expect.identical(notNull, list[2], "test41");
+  Expect.identical(theNull, list[3], "test42");
+}