Implement the new '?.' operator in analyzer.

tests/language/conditional_property_assignment_test.dart

Index: tests/language/conditional_property_assignment_test.dart
diff --git a/tests/language/conditional_property_assignment_test.dart b/tests/language/conditional_property_assignment_test.dart
new file mode 100644
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+++ b/tests/language/conditional_property_assignment_test.dart
@@ -0,0 +1,93 @@
+// 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 on the LHS of an
+// assignment.
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+import "conditional_access_helper.dart" as h;
+
+bad() {
+  Expect.fail('Should not be executed');
+}
+
+noMethod(e) => e is NoSuchMethodError;
+
+class B {}
+
+class C extends B {
+  int v;
+  C(this.v);
+  static var staticField;
+}
+
+class D {
+  E v;
+  D(this.v);
+}
+
+class E {
+  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 = e2 is equivalent to ((x) => x == null ? null : x.v = e2)(e1).
+  Expect.equals(null, nullC()?.v = bad()); /// 01: ok
+  { C c = new C(1); Expect.equals(2, c?.v = 2); Expect.equals(2, c.v); } /// 02: ok
+
+  // The static type of e1?.v = e2 is the static type of e2.
+  { D d = new D(new E()); G g = new G(); F f = (d?.v = g); Expect.identical(f, g); } /// 03: ok
+  { D d = new D(new E()); E e = new G(); F f = (d?.v = e); Expect.identical(f, e); } /// 04: static type warning
+
+  // Exactly the same static warnings that would be caused by e1.v = e2 are
+  // also generated in the case of e1?.v = e2.
+  Expect.equals(null, nullC()?.bad = bad()); /// 05: static type warning
+  { B b = new C(1); Expect.equals(2, b?.v = 2); Expect.equals(2, (b as C).v); } /// 06: static type warning
+
+  // e1?.v op= e2 is equivalent to ((x) => x?.v = x.v op e2)(e1).
+  Expect.equals(null, nullC()?.v += bad()); /// 07: ok
+  { C c = new C(1); Expect.equals(3, c?.v += 2); Expect.equals(3, c.v); } /// 08: ok
+
+  // The static type of e1?.v op= e2 is the static type of e1.v op e2.
+  { D d = new D(new E()); F f = (d?.v += 1); Expect.identical(d.v, f); } /// 09: ok
+
+  // Let T be the static type of e1 and let y be a fresh variable of type T.
+  // Exactly the same static warnings that would be caused by y.v op e2 are
+  // also generated in the case of e1?.v op= e2.
+  Expect.equals(null, nullC()?.bad = bad()); /// 10: static type warning
+  { B b = new C(1); Expect.equals(3, b?.v += 2); Expect.equals(3, (b as C).v); } /// 11: static type warning
+  { D d = new D(new E()); F f = (d?.v += nullC()); Expect.identical(d.v, f); } /// 12: static type warning
+  { D d = new D(new E()); H h = (d?.v += 1); Expect.identical(d.v, h); } /// 13: static type warning
+
+  // Consequently, '?.' cannot be used to assign to static properties of
+  // classes.
+  Expect.throws(() => C?.staticField = null, noMethod); /// 14: static type warning
+  Expect.throws(() => C?.staticField += null, noMethod); /// 15: static type warning
+  Expect.throws(() => C?.staticField ??= null, noMethod); /// 16: static type warning
+  Expect.throws(() => h.C?.staticField = null, noMethod); /// 17: static type warning
+  Expect.throws(() => h.C?.staticField += null, noMethod); /// 18: static type warning
+  Expect.throws(() => h.C?.staticField ??= null, noMethod); /// 19: static type warning
+
+  // Nor can it be used to assign to toplevel properties in libraries imported
+  // via prefix.
+  Expect.throws(() => h?.topLevelVar = null, noMethod); /// 20: static type warning
+  Expect.throws(() => h?.topLevelVar += null, noMethod); /// 21: static type warning
+  Expect.throws(() => h?.topLevelVar ??= null, noMethod); /// 22: static type warning
+}