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

pkg/analyzer/test/generated/resolver_test.dart

Index: pkg/analyzer/test/generated/resolver_test.dart
diff --git a/pkg/analyzer/test/generated/resolver_test.dart b/pkg/analyzer/test/generated/resolver_test.dart
index 7ba6e181e6ac64c29e865b4904f67c8358d014cd..d3e9748b57d0cb35e82030381fc9be30d980ec97 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -9892,6 +9892,24 @@ class StaticTypeAnalyzerTest extends EngineTestCase {
     _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;