Fix exception in DefinitionSource (#1874)

* Remove stale reference

* Don't suppress LHS diagnostics on augmented assign

* Revert "Don't suppress LHS diagnostics on augmented assign"

This reverts commit 6109ac7.

* Escape [ and ]

* PR feedback

* Fix exception

Author: Mikhail Arkhipov

src/LanguageServer/Impl/Sources/DefinitionSource.cs

@@ -209,7 +209,7 @@ private Reference HandleImport(IDocumentAnalysis analysis, ImportStatement state
             }
 
             // Import A as B
-            var asName = statement.AsNames.FirstOrDefault(n => n.IndexSpan.Start <= expr.StartIndex && n.IndexSpan.Start <= expr.EndIndex);
+            var asName = statement.AsNames.ExcludeDefault().FirstOrDefault(n => n.IndexSpan.Start <= expr.StartIndex && n.IndexSpan.Start <= expr.EndIndex);
             if (asName != null) {
                 var value = analysis.ExpressionEvaluator.GetValueFromExpression(asName);
                 if (!value.IsUnknown()) {