getObject now supports specifying offset,count for strings.

runtime/vm/json_test.cc

Index: runtime/vm/json_test.cc
diff --git a/runtime/vm/json_test.cc b/runtime/vm/json_test.cc
index 245bdd681e201fe1cffeeb887245523853f54c01..985b547592b1428db3841ac11fd52a257de41264 100644
--- a/runtime/vm/json_test.cc
+++ b/runtime/vm/json_test.cc
@@ -236,9 +236,22 @@ TEST_CASE(JSON_JSONStream_DartString) {
     JSONStream js;
     {
       JSONObject jsobj(&js);
-      jsobj.AddPropertyStr("ascci", obj);;
+      EXPECT(!jsobj.AddPropertyStr("ascii", obj));
     }
-    EXPECT_STREQ("{\"ascci\":\"Hello, World!\"}", js.ToCString());
+    EXPECT_STREQ("{\"ascii\":\"Hello, World!\"}", js.ToCString());
+  }
+
+  {
+    result = Dart_GetField(lib, NewString("ascii"));
+    EXPECT_VALID(result);
+    obj ^= Api::UnwrapHandle(result);
+
+    JSONStream js;
+    {
+      JSONObject jsobj(&js);
+      EXPECT(jsobj.AddPropertyStr("subrange", obj, 1, 4));
+    }
+    EXPECT_STREQ("{\"subrange\":\"ello\"}", js.ToCString());
   }
 
   {
@@ -249,7 +262,7 @@ TEST_CASE(JSON_JSONStream_DartString) {
     JSONStream js;
     {
       JSONObject jsobj(&js);
-      jsobj.AddPropertyStr("unicode", obj);
+      EXPECT(!jsobj.AddPropertyStr("unicode", obj));
     }
     EXPECT_STREQ("{\"unicode\":\"\\u00CE\\u00F1\\u0163\\u00E9r\\u00F1\\u00E5"
                  "\\u0163\\u00EE\\u00F6\\u00F1\\u00E5\\u013C\\u00EE\\u017E"
@@ -264,7 +277,7 @@ TEST_CASE(JSON_JSONStream_DartString) {
     JSONStream js;
     {
       JSONObject jsobj(&js);
-      jsobj.AddPropertyStr("surrogates", obj);
+      EXPECT(!jsobj.AddPropertyStr("surrogates", obj));
     }
     EXPECT_STREQ("{\"surrogates\":\"\\uD834\\uDD1E\\uD834\\uDD1E\\uD834\\uDD1E"
                  "\\uD834\\uDD1E\\uD834\\uDD1E\"}", js.ToCString());
@@ -278,7 +291,7 @@ TEST_CASE(JSON_JSONStream_DartString) {
     JSONStream js;
     {
       JSONObject jsobj(&js);
-      jsobj.AddPropertyStr("nullInMiddle", obj);
+      EXPECT(!jsobj.AddPropertyStr("nullInMiddle", obj));
     }
     EXPECT_STREQ("{\"nullInMiddle\":\"This has\\u0000 four words.\"}",
                  js.ToCString());