Implement the new '?.' operator in analyzer.

tests/language/if_null_assignment_behavior_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, including order of operations, by
 // keeping track of the operations performed.
 
 // SharedOptions=--enable-null-aware-operators
 
 import "package:expect/expect.dart";
@@ skipping 176 matching lines @@
   check(1, () => f().v ??= bad(), ['f()', 'f().v']); /// 22: continued
   fValue = new C('f()'); yGetValue = 1;                          /// 23: ok
   check(1, () => f().v ??= y, ['f()', 'f().v', 'y', 'f().v=1']); /// 23: continued
 
   // e1[e2] ??= e3 is equivalent to
   // ((a, i) => ((x) => x == null ? a[i] = e3 : x)(a[i]))(e1, e2)
   xGetValue = new C('x'); yGetValue = 1; xGetValue.indexGetValue = 2; /// 24: ok
   check(2, () => x[y] ??= bad(), ['x', 'y', 'x[1]']);                 /// 24: continued
   xGetValue = new C('x'); yGetValue = 1; zGetValue = 2;          /// 25: ok
   check(2, () => x[y] ??= z, ['x', 'y', 'x[1]', 'z', 'x[1]=2']); /// 25: continued
+
+  // e1?.v ??= e2 is equivalent to ((x) => x == null ? null : x.v ??= e2)(e1).
+  check(null, () => x?.v ??= bad(), ['x']); /// 26: ok
+  xGetValue = new C('x'); xGetValue.vGetValue = 1; /// 27: ok
+  check(1, () => x?.v ??= bad(), ['x', 'x.v']);     /// 27: continued
+  xGetValue = new C('x'); yGetValue = 1;                 /// 28: ok
+  check(1, () => x?.v ??= y, ['x', 'x.v', 'y', 'x.v=1']); /// 28: continued
 }