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 9d9a8d02cd76995a0770e08f25359081c97c9bb2..0c45439bc9831c819fb7e9e33df2a5b52477c4d1 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -365,6 +365,8 @@ class AnalysisContextForTests extends AnalysisContextImpl {
         currentOptions.dart2jsHint != options.dart2jsHint ||
         (currentOptions.hint && !options.hint) ||
         currentOptions.preserveComments != options.preserveComments ||
+        currentOptions.enableNullAwareOperators !=
+            options.enableNullAwareOperators ||
         currentOptions.enableStrictCallChecks != options.enableStrictCallChecks;
     if (needsRecompute) {
       fail(
@@ -9916,6 +9918,14 @@ class StaticTypeAnalyzerTest extends EngineTestCase {
     _listener.assertNoErrors();
   }
 
+  void test_visitBinaryExpression_ifNull() {
+    // 1 ?? 1.5
+    Expression node = AstFactory.binaryExpression(
+        _resolvedInteger(1), TokenType.QUESTION_QUESTION, _resolvedDouble(1.5));
+    expect(_analyze(node), same(_typeProvider.numType));
+    _listener.assertNoErrors();
+  }
+
   void test_visitBinaryExpression_logicalAnd() {
     // false && true
     Expression node = AstFactory.binaryExpression(