Ensure TextRange fields survive compilation

Change-Id: I2d9462890696c56a47c7f4755d4cbd6e330a987e
diff --git a/polygerrit-ui/app/api/diff.ts b/polygerrit-ui/app/api/diff.ts
index ee579ff..905d6be 100644
--- a/polygerrit-ui/app/api/diff.ts
+++ b/polygerrit-ui/app/api/diff.ts
@@ -55,7 +55,7 @@
 /**
  * Represents a "generic" text range in the code (e.g. text selection)
  */
-interface TextRange {
+export declare interface TextRange {
   /** first line of the range (1-based inclusive). */
   start_line: number;
   /** first column of the range (in the first line) (1-based inclusive). */