Add indexer to ImmutableSortedSet`1+Builder class.

The indexer is available on the immutable type, so its absence from the Builder is notable. Fortunately, it's simple to correct.

New tests included.

src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableSortedSet`1+Builder.cs

@@ -97,6 +97,20 @@ namespace System.Collections.Immutable
 
             #endregion
 
+            /// <summary>
+            /// Gets the element of the set at the given index.
+            /// </summary>
+            /// <param name="index">The 0-based index of the element in the set to return.</param>
+            /// <returns>The element at the given position.</returns>
+            /// <remarks>
+            /// No index setter is offered because the element being replaced may not sort
+            /// to the same position in the sorted collection as the replacing element.
+            /// </remarks>
+            public T this[int index]
+            {
+                get { return this.root[index]; }
+            }
+
             /// <summary>
             /// Gets the maximum value in the collection, as defined by the comparer.
             /// </summary>

src/System.Collections.Immutable/tests/ImmutableSortedSetBuilderTest.cs

@@ -313,5 +313,17 @@ namespace System.Collections.Immutable.Test
             Assert.NotNull(builder.SyncRoot);
             Assert.Same(builder.SyncRoot, builder.SyncRoot);
         }
+
+        [Fact]
+        public void Indexer()
+        {
+            var builder = ImmutableSortedSet.Create(1, 3, 2).ToBuilder();
+            Assert.Equal(1, builder[0]);
+            Assert.Equal(2, builder[1]);
+            Assert.Equal(3, builder[2]);
+
+            Assert.Throws<ArgumentOutOfRangeException>(() => builder[-1]);
+            Assert.Throws<ArgumentOutOfRangeException>(() => builder[3]);
+        }
     }
 }