Handle null in augmentField

tests/compiler/dart2js/cpa_inference_test.dart

 // Copyright (c) 2012, 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.
 
 import "dart:uri";
 import "../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/types/types.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
@@ skipping 343 matching lines @@
         var x;
         A(this.x);
         get y => x;
         get z => y;
       }
       main() {
         var a = new A(42);
         var foo = a.x;
         var bar = a.y;
         var baz = a.z;
-        foo; bar; baz;
+        var qux = null.x;
+        foo; bar; baz; qux;
       }
       """;
   AnalysisResult result = analyze(source);
   result.checkNodeHasType('foo', [result.int]);
   result.checkNodeHasType('bar', [result.int]);
   result.checkNodeHasType('baz', [result.int]);
+  result.checkNodeHasType('qux', []);
 }
 
 testSetters() {
   final String source = r"""
       class A {
         var x;
         var w;
         A(this.x, this.w);
         set y(a) { x = a; z = a; }
         set z(a) { w = a; }
       }
       main() {
         var a = new A(42, 42);
         a.x = 'abc';
         a.y = true;
+        null.x = 42;  // should be ignored
       }
       """;
   AnalysisResult result = analyze(source);
   result.checkFieldHasType('A', 'x', [result.int, result.string, result.bool]);
   result.checkFieldHasType('A', 'w', [result.int, result.bool]);
 }
 
 testNamedParameters() {
   final String source = r"""
       class A {
@@ skipping 212 matching lines @@
   testListLiterals();
   testMapLiterals();
   testReturn();
   // testNoReturn(); // right now we infer the empty type instead of null
   testArithmeticOperators();
   testOperators();
   testCompoundOperators1();
   testCompoundOperators2();
   // testFieldInitialization(); // TODO(polux)
 }