Update src/Workspaces/CoreTest/SymbolKeyTests.cs

Co-authored-by: NJason Malinowski <jason@jason-m.com>

src/Workspaces/CoreTest/SymbolKeyTests.cs

@@ -902,7 +902,7 @@ void M()
             Assert.Equal(propType.Kind, found.Kind);
             Assert.Equal(propType.ContainingNamespace.Name, found.ContainingNamespace.Name);
 
-            var method = found as IErrorTypeSymbol;
+            var method = (IErrorTypeSymbol)found;
             Assert.True(SymbolEquivalenceComparer.Instance.Equals(propType, found));
         }