Dexie export import fixes (dexie#779)

* Typescript compilation error resolved by exporting TypeMapper

* Sporadically, the unit tests of dexie-export-import fails with FileError
This happens mostly on Safari but has also happened on other browser.
Changed the local version of promisedTest() to include the error code from FileError when it happens. Also showing the stack when there's a failure.

The result: FileError with error code 1 happens occationally on Safari when reading a Blob asynchronically (the Blob that resides in IndexedDB).

Due to this erratic behaviour of Blob reading from IndexedDB in Safari, we have to retry the unit tests of dexie-export-import when it fails (retry logic implemented in addons/dexie-export-import/test/travis.sh)

addons/dexie-export-import/src/helpers.ts

@@ -30,7 +30,7 @@ declare var FileReaderSync: {
 };
 // -----------------------------------------------
 
-interface TypeMapper {
+export interface TypeMapper {
   binary: ArrayBuffer;
   text: string;
 }

addons/dexie-export-import/test/tools.ts

@@ -5,7 +5,11 @@ export function promisedTest(name: string, tester: ()=>Promise<any>) {
     try {
       await tester();
     } catch (error) {
-      ok(false, "Got error: " + error);
+      ok(false, "Got error: " + (error ?
+        error +
+          (error.code ? ` (code: ${error.code})` : ``) + 
+          (error.stack ? "\n" + error.stack : '') :
+        error));
     } finally {
       start();
     }

addons/dexie-export-import/test/travis.sh

@@ -2,4 +2,12 @@
 echo "Installing dependencies for dexie-export-import"
 npm install >/dev/null
 npm run build
-npm run test
+# This test fails sporadically on Safari 12. Needs to retry it if it fails.
+n=1
+until [ $n -ge 4 ]
+do
+  echo "Retry $n of 3"
+  npm run test && exit 0
+  n=$[$n+1]
+done
+exit 1