Get the basics of deferred libraries working.

pkg/dev_compiler/lib/js/legacy/dart_sdk.js

Index: pkg/dev_compiler/lib/js/legacy/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
index c230e85fc6c15ed0655a7cf21cd40484d4096895..ba55c46c2f0ac9b00238ea62606ce8ed7d631910 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
@@ -1939,6 +1939,9 @@ dart_library.library('dart_sdk', null, /* Imports */[
     }
     return name;
   };
+  dart.loadLibrary = function() {
+    return async.Future.value();
+  };
   dart.defineProperty = function(obj, name, desc) {
     return Object.defineProperty(obj, name, desc);
   };