Dart: Better handle leak checks. close() is async.

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

 {%- for method in interface.methods %}
 const int k{{interface|name}}_{{method|name}}_name = {{method.ordinal}};
 {%- endfor %}
 
 const String {{interface|name}}Name =
       '{{namespace|replace(".","::")}}::{{interface|name}}';
 
 abstract class {{interface|name}} {
 {%- for method in interface.methods %}
 {%- if method.response_parameters == None %}
@@ skipping 136 matching lines @@
 
   {{interface|name}}Proxy.unbound() :
       impl = new {{interface|name}}ProxyImpl.unbound() {
     ptr = new _{{interface|name}}ProxyCalls(impl);
   }
 
   static {{interface|name}}Proxy newFromEndpoint(
       core.MojoMessagePipeEndpoint endpoint) =>
       new {{interface|name}}Proxy.fromEndpoint(endpoint);
 
-  void close() => impl.close();
+  Future close() => impl.close();
 
   String toString() {
     return "{{interface|name}}Proxy($impl)";
   }
 }
 
 
 class {{interface|name}}Stub extends bindings.Stub {
   {{interface|name}} _impl = null;
 
@@ skipping 72 matching lines @@
       set impl({{interface|name}} d) {
     assert(_impl == null);
     _impl = d;
   }
 
   String toString() {
     var superString = super.toString();
     return "{{interface|name}}Stub($superString)";
   }
 }