Implement the new '?.' operator in analyzer.

tests/language/conditional_method_invocation_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 is used to invoke a method.
+
+// 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 {
+  f(callback()) => callback();
+  int g(int callback()) => callback();
+}
+
+C nullC() => null;
+
+main() {
+  // Make sure the "none" test fails if method invocation using "?." is not
+  // implemented.  This makes status files easier to maintain.
+  nullC()?.f(null);
+
+  // o?.m(...) is equivalent to ((x) => x == null ? null : x.m(...))(o).
+  Expect.equals(null, nullC()?.f(bad())); /// 01: ok
+  Expect.equals(1, new C()?.f(() => 1)); /// 02: ok
+
+  // The static type of o?.m(...) is the same as the static type of
+  // o.m(...).
+  { int i = nullC()?.g(bad()); Expect.equals(null, i); } /// 03: ok
+  { int i = new C()?.g(() => 1); Expect.equals(1, i); } /// 04: ok
+  { String s = nullC()?.g(bad()); Expect.equals(null, s); } /// 05: static type warning
+  { String s = new C()?.g(() => null); Expect.equals(null, s); } /// 06: static type warning
+
+  // Let T be the static type of o and let y be a fresh variable of type T.
+  // Exactly the same static warnings that would be caused by y.m(...) are also
+  // generated in the case of o?.m(...).
+  { B b = new C(); Expect.equals(1, b?.f(() => 1)); } /// 07: static type warning
+  { int i = 1; Expect.equals(null, nullC()?.f(i)); } /// 08: static type warning
+
+  // Consequently, '?.' cannot be used to invoke static methods of classes.
+  Expect.throws(() => C?.staticMethod(), noMethod); /// 09: static type warning
+  Expect.throws(() => h.C?.staticMethod(), noMethod); /// 10: static type warning
+
+  // Nor can it be used to access toplevel functions in libraries imported via
+  // prefix.
+  Expect.throws(() => h?.topLevelFunction(), noMethod); /// 11: static type warning
+
+  // However, '?.' can be used to access the toString method on the class Type.
+  Expect.equals(C?.toString(), (C).toString()); /// 12: ok
+  Expect.equals(h.C?.toString(), (h.C).toString()); /// 13: ok
+}