C#: Add indexer data flow test.

csharp/ql/test/library-tests/dataflow/indexers/IndexerFlow.ql

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+/**
+ * @kind path-problem
+ */
+
+import csharp
+import utils.test.InlineFlowTest
+import DefaultFlowTest
+import PathGraph
+
+from PathNode source, PathNode sink
+where flowPath(source, sink)
+select sink, source, sink, "$@", source, source.toString()

csharp/ql/test/library-tests/dataflow/indexers/Indexers.cs

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+public partial class Partial1
+{
+    private object[] _backingArray = new object[10];
+    public partial object this[int index]
+    {
+        get { return _backingArray[index]; }
+        set { _backingArray[index] = value; }
+    }
+}
+
+public partial class Partial1
+{
+    public partial object this[int index] { get; set; }
+}
+
+public partial class Partial2
+{
+    public partial object this[int index]
+    {
+        get { return null; }
+        set { }
+    }
+}
+
+public partial class Partial2
+{
+    public partial object this[int index] { get; set; }
+}
+
+public partial class PartialTest
+{
+    public void M()
+    {
+        var o = Source<object>(1);
+
+        var p1 = new Partial1();
+        p1[0] = o;
+        Sink(p1[0]); // $ hasValueFlow=1
+
+        var p2 = new Partial2();
+        p2[0] = o;
+        Sink(p2[0]); // no flow
+    }
+
+    public static void Sink(object o) { }
+
+    static T Source<T>(object source) => throw null;
+}