Fix build/lint due to differences in master and LKG (#32450)

Author: andrewbranch

src/compiler/emitter.ts

@@ -411,7 +411,11 @@ namespace ts {
                     }
                 );
                 if (emitOnlyDtsFiles && declarationTransform.transformed[0].kind === SyntaxKind.SourceFile) {
-                    const sourceFile = declarationTransform.transformed[0] as SourceFile;
+                    // Improved narrowing in master/3.6 makes this cast unnecessary, triggering a lint rule.
+                    // But at the same time, the LKG (3.5) necessitates it because it doesn’t narrow.
+                    // Once the LKG is updated to 3.6, this comment, the cast to `SourceFile`, and the
+                    // tslint directive can be all be removed.
+                    const sourceFile = declarationTransform.transformed[0] as SourceFile; // tslint:disable-line
                     exportedModulesFromDeclarationEmit = sourceFile.exportedModulesFromDeclarationEmit;
                 }
             }