No longer store the compilation-context in WorkItem.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index aa9dcdacd4b3ff91539ba5124218d36b733a2969..7906b04f0590bb4bcce0c023a8d6478e34ba9901 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -34,7 +34,6 @@ import '../universe/world_impact.dart' show ImpactStrategy, WorldImpact;
 import 'codegen.dart' show CodegenWorkItem;
 import 'registry.dart' show Registry;
 import 'tasks.dart' show CompilerTask;
-import 'work.dart' show ItemCompilationContext;
 
 abstract class Backend extends Target {
   final Compiler compiler;
@@ -105,10 +104,6 @@ abstract class Backend extends Target {
   void onResolutionComplete() {}
   void onTypeInferenceComplete() {}
 
-  ItemCompilationContext createItemCompilationContext() {
-    return new ItemCompilationContext();
-  }
-
   bool classNeedsRti(ClassElement cls);
   bool methodNeedsRti(FunctionElement function);