extend asset request builder, add query assets test

lib/src/requests/assets_request_builder.dart

@@ -8,6 +8,8 @@ import '../responses/response.dart';
 import '../responses/asset_response.dart';
 import 'request_builder.dart';
 
+/// Lists all assets. It will give you all the assets in the system along with various statistics about each.
+/// See: https://developers.stellar.org/api/resources/assets/object/
 class AssetsRequestBuilder extends RequestBuilder {
   AssetsRequestBuilder(http.Client httpClient, Uri serverURI)
       : super(httpClient, serverURI, ["assets"]);
@@ -39,4 +41,22 @@ class AssetsRequestBuilder extends RequestBuilder {
     return AssetsRequestBuilder.requestExecute(
         this.httpClient, this.buildUri());
   }
+
+  @override
+  AssetsRequestBuilder cursor(String token) {
+    super.cursor(token);
+    return this;
+  }
+
+  @override
+  AssetsRequestBuilder limit(int number) {
+    super.limit(number);
+    return this;
+  }
+
+  @override
+  AssetsRequestBuilder order(RequestBuilderOrder direction) {
+    super.order(direction);
+    return this;
+  }
 }

test/query_test.dart

@@ -92,4 +92,31 @@ void main() {
         .execute();
     assert(accountsForAsset.records.length == 2);
   });
+
+  test('test query assets', () async {
+    Page<AssetResponse> assetsPage = await sdk.assets
+        .assetCode("USD")
+        .limit(5)
+        .order(RequestBuilderOrder.DESC)
+        .execute();
+    List<AssetResponse> assets = assetsPage.records;
+    assert(assets.length > 0 && assets.length < 6);
+    for (AssetResponse asset in assets) {
+      print("asset issuer: " + asset.assetIssuer);
+    }
+    String assetIssuer = assets.last.assetIssuer;
+    assetsPage = await sdk.assets
+        .assetIssuer(assetIssuer)
+        .limit(5)
+        .order(RequestBuilderOrder.DESC)
+        .execute();
+    assets = assetsPage.records;
+    assert(assets.length > 0 && assets.length < 6);
+    for (AssetResponse asset in assets) {
+      print("asset code: " +
+          asset.assetCode +
+          " amount:${asset.amount} " +
+          "num accounts:${asset.numAccounts}");
+    }
+  });
 }