Insert checks before deferred calls and accesses.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 2d0b7f5631e9cb2f681bb65912372b3db917f02a..bbf2b4279ce815c0c68714db7041f57427d74303 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1065,6 +1065,12 @@ class JavaScriptBackend extends Backend {
     enqueueInResolution(getFallThroughError(), elements);
   }
 
+  void registerCheckDeferredIsLoaded(TreeElements elements) {
+    enqueueInResolution(getCheckDeferredIsLoaded(), elements);
+    // Also register the types of the arguments passed to this method.
+    enqueueClass(compiler.enqueuer.resolution, compiler.stringClass, elements);
+  }
+
   void enableNoSuchMethod(Enqueuer world) {
     enqueue(world, getCreateInvocationMirror(), compiler.globalDependencies);
     world.registerInvocation(compiler.noSuchMethodSelector);
@@ -1526,6 +1532,10 @@ class JavaScriptBackend extends Backend {
     return compiler.findHelper('checkSubtypeOfRuntimeType');
   }
 
+  Element getCheckDeferredIsLoaded() {
+    return compiler.findHelper('checkDeferredIsLoaded');
+  }
+
   Element getAssertSubtypeOfRuntimeType() {
     return compiler.findHelper('assertSubtypeOfRuntimeType');
   }