implement null aware ops, fixes #249

lib/runtime/dart/_internal.js

Index: lib/runtime/dart/_internal.js
diff --git a/lib/runtime/dart/_internal.js b/lib/runtime/dart/_internal.js
index b77e4578d05de7ce723293f54b47341ffc984642..8f2e5dd0c6e7d42ceec2f59460d3d4999b01ca71 100644
--- a/lib/runtime/dart/_internal.js
+++ b/lib/runtime/dart/_internal.js
@@ -1655,6 +1655,7 @@ dart_library.library('dart/_internal', null, /* Imports */[
       set(index, value) {
         dart.as(value, E);
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable list"));
+        return value;
       }
       set length(newLength) {
         dart.throw(new core.UnsupportedError("Cannot change the length of an unmodifiable list"));
@@ -1857,6 +1858,7 @@ dart_library.library('dart/_internal', null, /* Imports */[
       set(key, value) {
         dart.as(value, E);
         dart.throw(new core.UnsupportedError("Cannot modify an unmodifiable map"));
+        return value;
       }
       putIfAbsent(key, ifAbsent) {
         dart.as(ifAbsent, dart.functionType(E, []));