Do not crash when highlighting alias symbols defined via global imports.

Fixes #1904

src/EditorFeatures/Core/Implementation/ReferenceHighlighting/AbstractDocumentHighlightsService.cs

@@ -130,12 +130,17 @@ private bool ShouldConsiderSymbol(ISymbol symbol)
             bool addAllDefinitions = true;
 
             // Add definitions
+            // Filter out definitions that cannot be highlighted. e.g: alias symbols defined via project property pages.
             if (symbol.Kind == SymbolKind.Alias &&
                 symbol.Locations.Length > 0)
             {
-                // For alias symbol we want to get the tag only for the alias definition, not the target symbol's definition.
-                await AddLocationSpan(symbol.Locations.First(), solution, spanSet, tagMap, HighlightSpanKind.Definition, cancellationToken).ConfigureAwait(false);
                 addAllDefinitions = false;
+
+                if (symbol.Locations.First().IsInSource)
+                {
+                    // For alias symbol we want to get the tag only for the alias definition, not the target symbol's definition.
+                    await AddLocationSpan(symbol.Locations.First(), solution, spanSet, tagMap, HighlightSpanKind.Definition, cancellationToken).ConfigureAwait(false);
+                }
             }
 
             // Add references and definitions

src/EditorFeatures/Test2/ReferenceHighlighting/VisualBasicReferenceHighlightingTests.vb

@@ -272,5 +272,27 @@ End Class
 
             VerifyHighlights(input)
         End Sub
+
+        <WorkItem(1904, "https://github.com/dotnet/roslyn/issues/1904")>
+        <WorkItem(2079, "https://github.com/dotnet/roslyn/issues/2079")>
+        <Fact, Trait(Traits.Feature, Traits.Features.ReferenceHighlighting)>
+        Public Sub VerifyHighlightsForVisualBasicGlobalImportAliasedNamespace()
+            VerifyHighlights(
+                <Workspace>
+                    <Project Language="Visual Basic" CommonReferences="true">
+                        <CompilationOptions><GlobalImport>VB = Microsoft.VisualBasic</GlobalImport></CompilationOptions>
+                        <Document>
+                            Class Test
+                                Public Sub TestMethod()
+                                    ' Add reference tags to verify after #2079 is fixed
+                                    Console.Write(NameOf($$VB))
+                                    Console.Write(NameOf(VB))
+                                    Console.Write(NameOf(Microsoft.VisualBasic))
+                                End Sub
+                            End Class
+                        </Document>
+                    </Project>
+                </Workspace>)
+        End Sub
     End Class
 End Namespace