Restore unit test

src/EditorFeatures/CSharpTest/ChangeSignature/RemoveParametersTests.cs

@@ -184,6 +184,91 @@ void B()
             await TestChangeSignatureViaCommandAsync(LanguageNames.CSharp, markup, updatedSignature: updatedSignature, expectedUpdatedInvocationDocumentCode: updatedCode);
         }
 
+        [WpfFact, Trait(Traits.Feature, Traits.Features.ChangeSignature)]
+        [WorkItem(1102830, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/1102830")]
+        [WorkItem(784, "https://github.com/dotnet/roslyn/issues/784")]
+        public async Task RemoveParameters_ExtensionMethodInAnotherFile()
+        {
+            var workspaceXml = @"
+<Workspace>
+    <Project Language=""C#"" AssemblyName=""CSharpAssembly"" CommonReferences=""true"">";
+
+            for (var i = 0; i <= 4; i++)
+            {
+                workspaceXml += $@"
+<Document FilePath = ""C{i}.cs"">
+class C{i}
+{{
+    void M()
+    {{
+        C5 c = new C5();
+        c.Ext(1, ""two"");
+    }}
+}}
+</Document>";
+            }
+
+            workspaceXml += @"
+<Document FilePath = ""C5.cs"">
+public class C5
+{
+}
+
+public static class C5Ext
+{
+    public void $$Ext(this C5 c, int i, string s)
+    {
+    }
+}
+</Document>";
+
+            for (var i = 6; i <= 9; i++)
+            {
+                workspaceXml += $@"
+<Document FilePath = ""C{i}.cs"">
+class C{i}
+{{
+    void M()
+    {{
+        C5 c = new C5();
+        c.Ext(1, ""two"");
+    }}
+}}
+</Document>";
+            }
+
+            workspaceXml += @"
+    </Project>
+</Workspace>";
+
+            // Ext(this F f, int i, string s) --> Ext(this F f, string s)
+            // If a reference does not bind correctly, it will believe Ext is not an extension
+            // method and remove the string argument instead of the int argument.
+
+            var updatedSignature = new[] {
+                new AddedParameterOrExistingIndex(0),
+                new AddedParameterOrExistingIndex(2) };
+
+            using var testState = ChangeSignatureTestState.Create(XElement.Parse(workspaceXml));
+            testState.TestChangeSignatureOptionsService.UpdatedSignature = updatedSignature;
+            var result = testState.ChangeSignature();
+
+            Assert.True(result.Succeeded);
+            Assert.Null(testState.ErrorMessage);
+
+            foreach (var updatedDocument in testState.Workspace.Documents.Select(d => result.UpdatedSolution.GetDocument(d.Id)))
+            {
+                if (updatedDocument.Name == "C5.cs")
+                {
+                    Assert.Contains("void Ext(this C5 c, string s)", (await updatedDocument.GetTextAsync(CancellationToken.None)).ToString());
+                }
+                else
+                {
+                    Assert.Contains(@"c.Ext(""two"");", (await updatedDocument.GetTextAsync(CancellationToken.None)).ToString());
+                }
+            }
+        }
+
         [WpfFact, Trait(Traits.Feature, Traits.Features.ChangeSignature)]
         [WorkItem(1102830, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/1102830")]
         [WorkItem(784, "https://github.com/dotnet/roslyn/issues/784")]