Fix linting

Author: jack-williams

src/compiler/emitter.ts

@@ -411,7 +411,7 @@ namespace ts {
                     }
                 );
                 if (emitOnlyDtsFiles && declarationTransform.transformed[0].kind === SyntaxKind.SourceFile) {
-                    const sourceFile = declarationTransform.transformed[0] as SourceFile;
+                    const sourceFile = declarationTransform.transformed[0];
                     exportedModulesFromDeclarationEmit = sourceFile.exportedModulesFromDeclarationEmit;
                 }
             }