Rewrite asyncStepOver to use await

runtime/observatory/tests/service/get_object_rpc_test.dart

Index: runtime/observatory/tests/service/get_object_rpc_test.dart
diff --git a/runtime/observatory/tests/service/get_object_rpc_test.dart b/runtime/observatory/tests/service/get_object_rpc_test.dart
index 2fc74f497555ea6edfec58230fcacee9e230a7d4..a98c53598eae7f07f3b5dfa91ada15b55f4736c4 100644
--- a/runtime/observatory/tests/service/get_object_rpc_test.dart
+++ b/runtime/observatory/tests/service/get_object_rpc_test.dart
@@ -448,7 +448,7 @@ var tests = [
     expect(result['offset'], isNull);
     expect(result['count'], isNull);
     expect(result['bytes'], equals('AwIB'));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint8List().toString(), equals('[3, 2, 1]'));
   },
 
@@ -474,7 +474,7 @@ var tests = [
     expect(result['offset'], isNull);
     expect(result['count'], equals(2));
     expect(result['bytes'], equals('AwI='));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint8List().toString(), equals('[3, 2]'));
   },
 
@@ -501,7 +501,7 @@ var tests = [
     expect(result['offset'], equals(2));
     expect(result['count'], equals(1));
     expect(result['bytes'], equals('AQ=='));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint8List().toString(), equals('[1]'));
   },
 
@@ -551,7 +551,7 @@ var tests = [
     expect(result['offset'], isNull);
     expect(result['count'], isNull);
     expect(result['bytes'], equals('AwAAAAAAAAACAAAAAAAAAAEAAAAAAAAA'));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint64List().toString(), equals('[3, 2, 1]'));
   },
 
@@ -577,7 +577,7 @@ var tests = [
     expect(result['offset'], isNull);
     expect(result['count'], equals(2));
     expect(result['bytes'], equals('AwAAAAAAAAACAAAAAAAAAA=='));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint64List().toString(), equals('[3, 2]'));
   },
 
@@ -604,7 +604,7 @@ var tests = [
     expect(result['offset'], equals(2));
     expect(result['count'], equals(1));
     expect(result['bytes'], equals('AQAAAAAAAAA='));
-    var bytes = BASE64.decode(result['bytes']);
+    Uint8List bytes = BASE64.decode(result['bytes']);
     expect(bytes.buffer.asUint64List().toString(), equals('[1]'));
   },