Add tests for e?.v++, e?.v--, ++e?.v, and --e?.v.

tests/language/conditional_property_increment_decrement_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.
+
+// Verify semantics of the ?. operator when it appears in a postincrement or
+// preincrement expression (or a postdecrement or predecrement expression).
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+
+class C {
+  int v;
+  C(this.v);
+  static var staticField;
+}
+
+class D {
+  E v;
+  D(this.v);
+}
+
+class E {
+  G operator+(int i) => new I();
+  G operator-(int i) => new I();
+}
+
+class F {}
+
+class G extends E implements F {}
+
+class H {}
+
+class I extends G implements H {}
+
+C nullC() => null;
+
+main() {
+  // Make sure the "none" test fails if assignment to "?." is not implemented.
+  // This makes status files easier to maintain.
+  nullC()?.v = 1;
+
+  // e1?.v++ is equivalent to ((x) => x == null ? null : x.v++)(e1).
+  Expect.equals(null, nullC()?.v++); /// 01: ok
+  { C c = new C(1); Expect.equals(1, c?.v++); Expect.equals(2, c.v); } /// 02: ok
+
+  // The static type of e1?.v++ is the same as the static type of e1.v.
+  { E e1 = new E(); D d = new D(e1); E e2 = d?.v++; Expect.identical(e1, e2); } /// 03: ok
+  { G g = new G(); D d = new D(g); F f = d?.v++; Expect.identical(f, g); } /// 04: static type warning
+
+  // e1?.v-- is equivalent to ((x) => x == null ? null : x.v--)(e1).
+  Expect.equals(null, nullC()?.v--); /// 05: ok
+  { C c = new C(1); Expect.equals(1, c?.v--); Expect.equals(0, c.v); } /// 06: ok
+
+  // The static type of e1?.v-- is the same as the static type of e1.v.
+  { E e1 = new E(); D d = new D(e1); E e2 = d?.v--; Expect.identical(e1, e2); } /// 07: ok
+  { G g = new G(); D d = new D(g); F f = d?.v--; Expect.identical(f, g); } /// 08: static type warning
+
+  // ++e1?.v is equivalent to e1?.v += 1.
+  Expect.equals(null, ++nullC()?.v); /// 09: ok
+  { C c = new C(1); Expect.equals(2, ++c?.v); Expect.equals(2, c.v); } /// 10: ok
+
+  // The static type of ++e1?.v is the same as the static type of e1.v + 1.
+  { D d = new D(new E()); F f = ++d?.v; Expect.identical(d.v, f); } /// 11: ok
+  { D d = new D(new E()); H h = ++d?.v; Expect.identical(d.v, h); } /// 12: static type warning
+
+  // --e1?.v is equivalent to e1?.v += 1.
+  Expect.equals(null, --nullC()?.v); /// 13: ok
+  { C c = new C(1); Expect.equals(0, --c?.v); Expect.equals(0, c.v); } /// 14: ok
+
+  // The static type of --e1?.v is the same as the static type of e1.v - 1.
+  { D d = new D(new E()); F f = --d?.v; Expect.identical(d.v, f); } /// 15: ok
+  { D d = new D(new E()); H h = --d?.v; Expect.identical(d.v, h); } /// 16: static type warning
+}