Implement the new '?.' operator in analyzer.

tests/language/conditional_property_access_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 does not appear on the LHS of an
+// assignment.
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+import "conditional_access_helper.dart" as h;
+
+noMethod(e) => e is NoSuchMethodError;
+
+class B {}
+
+class C extends B {
+  int v;
+  C(this.v);
+  static var staticField;
+}
+
+C nullC() => null;
+
+main() {
+  // Make sure the "none" test fails if property access using "?." is not
+  // implemented.  This makes status files easier to maintain.
+  nullC()?.v;
+
+  // e1?.id is equivalent to ((x) => x == null ? null : x.id)(e1).
+  Expect.equals(null, nullC()?.v); /// 01: ok
+  Expect.equals(1, new C(1)?.v); /// 02: ok
+
+  // The static type of e1?.d is the static type of e1.id.
+  { int i = new C(1)?.v; Expect.equals(1, i); } /// 03: ok
+  { String s = new C(null)?.v; Expect.equals(null, s); } /// 04: static type warning
+
+  // 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.id are also
+  // generated in the case of e1?.id.
+  Expect.equals(null, nullC()?.bad); /// 05: static type warning
+  { B b = new C(1); Expect.equals(1, b?.v); } /// 06: static type warning
+
+  // Consequently, '?.' cannot be used to access static properties of classes.
+  Expect.throws(() => C?.staticField, noMethod); /// 07: static type warning
+  Expect.throws(() => h.C?.staticField, noMethod); /// 08: static type warning
+
+  // Nor can it be used to access toplevel properties in libraries imported via
+  // prefix.
+  Expect.throws(() => h?.topLevelVar, noMethod); /// 09: static type warning
+
+  // However, '?.' can be used to access the hashCode getter on the class Type.
+  Expect.equals(C?.hashCode, (C).hashCode); /// 10: ok
+  Expect.equals(h.C?.hashCode, (h.C).hashCode); /// 11: ok
+}