Implement the new '??=' operator in analyzer.

pkg/analyzer/test/generated/resolver_test.dart

 // Copyright (c) 2014, 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.
 
 library engine.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 9874 matching lines @@
     InterfaceType numType = _typeProvider.numType;
     SimpleIdentifier identifier = _resolvedVariable(_typeProvider.intType, "i");
     AssignmentExpression node = AstFactory.assignmentExpression(
         identifier, TokenType.PLUS_EQ, _resolvedInteger(1));
     MethodElement plusMethod = getMethod(numType, "+");
     node.staticElement = plusMethod;
     expect(_analyze(node), same(numType));
     _listener.assertNoErrors();
   }
 
+  void test_visitAssignmentExpression_compoundIfNull_differentTypes() {
+    // double d; d ??= 0
+    Expression node = AstFactory.assignmentExpression(
+        _resolvedVariable(_typeProvider.doubleType, 'd'),
+        TokenType.QUESTION_QUESTION_EQ, _resolvedInteger(0));
+    expect(_analyze(node), same(_typeProvider.numType));
+    _listener.assertNoErrors();
+  }
+
+  void test_visitAssignmentExpression_compoundIfNull_sameTypes() {
+    // int i; i ??= 0
+    Expression node = AstFactory.assignmentExpression(
+        _resolvedVariable(_typeProvider.intType, 'i'),
+        TokenType.QUESTION_QUESTION_EQ, _resolvedInteger(0));
+    expect(_analyze(node), same(_typeProvider.intType));
+    _listener.assertNoErrors();
+  }
+
   void test_visitAssignmentExpression_simple() {
     // i = 0
     InterfaceType intType = _typeProvider.intType;
     Expression node = AstFactory.assignmentExpression(
         _resolvedVariable(intType, "i"), TokenType.EQ, _resolvedInteger(0));
     expect(_analyze(node), same(intType));
     _listener.assertNoErrors();
   }
 
   void test_visitAwaitExpression_flattened() {
@@ skipping 3862 matching lines @@
         // check propagated type
         FunctionType propagatedType = node.propagatedType as FunctionType;
         expect(propagatedType.returnType, test.typeProvider.stringType);
       } on AnalysisException catch (e, stackTrace) {
         thrownException[0] = new CaughtException(e, stackTrace);
       }
     }
     return null;
   }
 }