Raise an exception when converting a DartWrappable with empty native fields

Fixes https://github.com/flutter/flutter/issues/2391

sky/engine/tonic/dart_error.cc

@@ -9,6 +9,7 @@
 namespace blink {
 namespace DartError {
 const char kInvalidArgument[] = "Invalid argument.";
+const char kInvalidDartWrappable[] = "Invalid Dart wrappable object.";
 }  // namespace DartError
 
 bool LogIfError(Dart_Handle handle) {

sky/engine/tonic/dart_error.h

@@ -11,6 +11,7 @@ namespace blink {
 
 namespace DartError {
 extern const char kInvalidArgument[];
+extern const char kInvalidDartWrappable[];
 }  // namespace DartError
 
 bool LogIfError(Dart_Handle handle);

sky/engine/tonic/dart_wrappable.cc

@@ -101,6 +101,10 @@ DartWrappable* DartConverterWrappable::FromArguments(Dart_NativeArguments args,
     exception = Dart_NewStringFromCString(DartError::kInvalidArgument);
     return nullptr;
   }
+  if (!native_fields[DartWrappable::kPeerIndex]) {
+    exception = Dart_NewStringFromCString(DartError::kInvalidDartWrappable);
+    return nullptr;
+  }
   return reinterpret_cast<DartWrappable*>(
       native_fields[DartWrappable::kPeerIndex]);
 }