Allow Observatory debugger to switch isolates.

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index 388c0d327d330ee0b6800acc81ee737cf5eae971..d51acbb4029b6de51644993a39d265e4fda3c1b9 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -274,12 +274,14 @@ void JSONStream::PrintPropertyBool(const char* name, bool b) {
 
 
 void JSONStream::PrintProperty(const char* name, intptr_t i) {
+  ASSERT(Utils::IsJavascriptInt(i));
   PrintPropertyName(name);
   PrintValue(i);
 }
 
 
 void JSONStream::PrintProperty64(const char* name, int64_t i) {
+  ASSERT(Utils::IsJavascriptInt(i));
   PrintPropertyName(name);
   PrintValue64(i);
 }