Migration: implement post-dominator analysis of ?? operations.

Change-Id: I66ed10fba630663e025cec8d805ed6677b2eb6ff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114555
Reviewed-by: Mike Fairhurst <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -330,7 +330,10 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       _flowAnalysis.ifNullExpression_rightBegin();
       try {
         _guards.add(leftType.node);
-        var rightType = node.rightOperand.accept(this);
+        DecoratedType rightType;
+        _postDominatedLocals.doScoped(action: () {
+          rightType = node.rightOperand.accept(this);
+        });
         var ifNullNode = NullabilityNode.forIfNotNull();
         expressionType = DecoratedType(node.staticType, ifNullNode);
         _connect(rightType.node, expressionType.node,

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -2982,6 +2982,22 @@ void test() {
     assertEdge(always, decoratedTypeAnnotation('int i').node, hard: false);
   }
 
+  test_postDominators_questionQuestionOperator() async {
+    await analyze('''
+class C {
+  Object m() => null;
+}
+Object test(C x, C y) => x.m() ?? y.m();
+''');
+    // There is a hard edge from x to `never` because `x.m()` is unconditionally
+    // reachable from the top of `test`.
+    assertEdge(decoratedTypeAnnotation('C x').node, never, hard: true);
+    // However, the edge from y to `never` is soft because `y.m()` is only
+    // executed if `x.m()` returned `null`.
+    assertEdge(decoratedTypeAnnotation('C y').node, never,
+        hard: false, guards: [decoratedTypeAnnotation('Object m').node]);
+  }
+
   test_postDominators_reassign() async {
     await analyze('''
 void test(bool b, int i1, int i2) {