[nnbd_migration] Fix ConditionalModification on IfElement

Change-Id: I1e8ec4bead2aaa9147850d27cd5ac4d9c2a700be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/130137
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>

pkg/analyzer/lib/src/test_utilities/find_node.dart

@@ -51,6 +51,10 @@ class FindNode {
     return _node(search, (n) => n is ClassDeclaration);
   }
 
+  CollectionElement collectionElement(String search) {
+    return _node(search, (n) => n is CollectionElement);
+  }
+
   CommentReference commentReference(String search) {
     return _node(search, (n) => n is CommentReference);
   }

pkg/nnbd_migration/lib/src/potential_modification.dart

@@ -37,6 +37,15 @@ class ConditionalModification extends PotentialModification {
           _KeepNode(node.condition),
           _KeepNode(node.thenStatement),
           node.elseStatement == null ? null : _KeepNode(node.elseStatement));
+    } else if (node is IfElement) {
+      return ConditionalModification._(
+          node.offset,
+          node.end,
+          node is Statement,
+          discard,
+          _KeepNode(node.condition),
+          _KeepNode(node.thenElement),
+          node.elseElement == null ? null : _KeepNode(node.elseElement));
     } else {
       throw new UnimplementedError('TODO(paulberry)');
     }

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -2812,9 +2812,7 @@ void f(bool b, int i, int j) {
     assertEdge(decoratedTypeAnnotation('int j').node, never, hard: true);
   }
 
-  @failingTest
   Future<void> test_if_element_guard_equals_null() async {
-    // failing because of an unimplemented exception in conditional modification
     await analyze('''
 dynamic f(int i, int j, int k) {
   <int>[if (i == null) j/*check*/ else k/*check*/];
@@ -2830,7 +2828,7 @@ dynamic f(int i, int j, int k) {
             guards: [nullable_i], hard: false));
     assertNullCheck(checkExpression('k/*check*/'),
         assertEdge(nullable_k, nullable_itemType, hard: false));
-    var discard = statementDiscard('if (i == null)');
+    var discard = elementDiscard('if (i == null)');
     expect(discard.trueGuard, same(nullable_i));
     expect(discard.falseGuard, null);
     expect(discard.pureCondition, true);

pkg/nnbd_migration/test/migration_visitor_test_base.dart

@@ -403,6 +403,12 @@ class MigrationVisitorTestBase extends AbstractSingleUnitTest with EdgeTester {
   ConditionalDiscard statementDiscard(String text) {
     return variables.conditionalDiscard(findNode.statement(text));
   }
+
+  /// Gets the [ConditionalDiscard] information associated with the collection
+  /// element whose text is [text].
+  ConditionalDiscard elementDiscard(String text) {
+    return variables.conditionalDiscard(findNode.collectionElement(text));
+  }
 }
 
 /// Abstract base class representing a thing that can be matched against