Move global dependencies to BackendUsage

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 57db6e9c9e56f6d90dbcfc0b7b2433674230bc42..6916c5e8370fc30b1e33869133c3aaf38607a12d 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -680,8 +680,7 @@ class DeferredLoadTask extends CompilerTask {
               // are things that the backend needs but cannot associate with a
               // particular element, for example, startRootIsolate.  This set
               // also contains elements for which we lack precise information.
-              for (Element element
-                  in compiler.globalDependencies.otherDependencies) {
+              for (Element element in backend.backendUsage.globalDependencies) {
                 _mapDependencies(element: element, import: _fakeMainImport);
               }