Implement the new '?.' operator in analyzer.

tests/language/if_null_assignment_static_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 that the static type of a ??= b is the least upper bound of the
 // static types of a and b.
 
 // SharedOptions=--enable-null-aware-operators
 
 import "package:expect/expect.dart";
@@ skipping 139 matching lines @@
   ((<A>[null])[0] ??= new B()).a; /// 31: ok
   ((<A>[null])[0] ??= new B()).b; /// 32: static type warning
   if (!checkedMode) {
     ((<B>[null])[0] ??= new A()).a; /// 33: ok
     Expect.throws(() => ((<B>[null])[0] ??= new A()).b, noMethod); /// 34: static type warning
 
     // Exactly the same static warnings that would be caused by e1[e2] = e3 are
     // also generated in the case of e1[e2] ??= e3.
     (<B>[null])[0] ??= new C(); /// 35: static type warning
   }
+
+  // The static type of e1?.v op= e2 is the static type of e1.v op e2,
+  // therefore the static type of e1?.v ??= e2 is the static type of
+  // e1.v ?? e2, which is the LUB of the static types of e1?.v and e2.
+  (new ClassWithInstanceGetters()?.a ??= new A()).a; /// 36: ok
+  Expect.throws(() => (new ClassWithInstanceGetters()?.a ??= new A()).b, noMethod); /// 37: static type warning
+  (new ClassWithInstanceGetters()?.a ??= new B()).a; /// 38: ok
+  (new ClassWithInstanceGetters()?.a ??= new B()).b; /// 39: static type warning
+  if (!checkedMode) {
+    (new ClassWithInstanceGetters()?.b ??= new A()).a; /// 40: ok
+    Expect.throws(() => (new ClassWithInstanceGetters()?.b ??= new A()).b, noMethod); /// 41: 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.
+    new ClassWithInstanceGetters()?.b ??= new C(); /// 42: static type warning
+  }
 }