Fix broken builds under strict-function-types.

PiperOrigin-RevId: 201995415
diff --git a/internal/tsetse/checker.ts b/internal/tsetse/checker.ts
index 04c9ed3..322b22b 100644
--- a/internal/tsetse/checker.ts
+++ b/internal/tsetse/checker.ts
@@ -59,6 +59,9 @@
   on(nodeKind: ts.SyntaxKind.ElementAccessExpression,
      handlerFunction: (checker: Checker, node: ts.ElementAccessExpression) => void,
      code: number): void;
+  on(nodeKind: ts.SyntaxKind.ImportDeclaration,
+     handlerFunction: (checker: Checker, node: ts.ImportDeclaration) => void,
+     code: number): void;
   on(nodeKind: ts.SyntaxKind,
      handlerFunction: (checker: Checker, node: ts.Node) => void,
      code: number): void;