Dart: Merge message pipe query and read into one native call

mojo/public/tools/bindings/generators/dart_templates/struct_definition.tmpl

 {#--- Begin #}
 
 {%- import "encoding_macros.tmpl" as encoding_macros %}
 
 {%- macro struct_def(struct) %}
 {#--- Enums #}
 {%- from "enum_definition.tmpl" import enum_def %}
 {%-  for enum in struct.enums %}
 {{enum_def(enum)}}
 {%-  endfor %}
@@ skipping 13 matching lines @@
 {#--- initDefaults() #}
 {%- for packed_field in struct.packed.packed_fields %}
   {{packed_field.field.kind|dart_type}} {{packed_field.field|name}} = {{packed_field.field|default_value}};
 {%- endfor %}
 
   {{struct|name}}() : super(kVersions.last.size);
 
   static {{struct|name}} deserialize(bindings.Message message) {
     var decoder = new bindings.Decoder(message);
     var result = decode(decoder);
-    decoder.excessHandles.forEach((h) => h.close());
+    if (decoder.excessHandles != null) {
+      decoder.excessHandles.forEach((h) => h.close());
+    }
     return result;
   }
 
   static {{struct|name}} decode(bindings.Decoder decoder0) {
     if (decoder0 == null) {
       return null;
     }
     {{struct|name}} result = new {{struct|name}}();
 
     var mainDataHeader = decoder0.decodeStructDataHeader();
@@ skipping 52 matching lines @@
     map["{{packed_field.field|name}}"] = {{packed_field.field|name}};
     {%- endfor %}
     return map;
     {%- else %}
     throw new bindings.MojoCodecError(
         'Object containing handles cannot be encoded to JSON.');
     {%- endif %}
   }
 }
 {%- endmacro %}