fix: use ~ for typescript (and fix compile errors) (#426)

Author: JustinBeckwith

handwritten/storage/package.json

@@ -138,7 +138,7 @@
     "proxyquire": "^2.0.0",
     "source-map-support": "^0.5.6",
     "tmp": "^0.0.33",
-    "typescript": "^3.0.1",
+    "typescript": "~3.1.1",
     "uuid": "^3.1.0"
   }
 }

handwritten/storage/system-test/storage.ts

@@ -1384,7 +1384,7 @@ describe('storage', () => {
     it('should handle non-network errors', done => {
       const file = bucket.file('hi.jpg');
       file.download(err => {
-        assert.strictEqual(err.code, 404);
+        assert.strictEqual((err as ApiError).code, 404);
         done();
       });
     });
@@ -1593,7 +1593,7 @@ describe('storage', () => {
 
       it('should not download from the unencrypted file', done => {
         unencryptedFile.download(err => {
-          assert(err.message.indexOf([
+          assert(err!.message.indexOf([
             'The target object is encrypted by a',
             'customer-supplied encryption key.',
           ].join(' ')) > -1);