Decouple WorkItem from Compiler

pkg/compiler/lib/src/universe/use.dart

Index: pkg/compiler/lib/src/universe/use.dart
diff --git a/pkg/compiler/lib/src/universe/use.dart b/pkg/compiler/lib/src/universe/use.dart
index ce9a2c4f6ddda7f1e2d27626c5feeda452159c9e..f3995098744c130979e15643a54da6522caf3855 100644
--- a/pkg/compiler/lib/src/universe/use.dart
+++ b/pkg/compiler/lib/src/universe/use.dart
@@ -75,6 +75,7 @@ enum StaticUseKind {
   CONST_CONSTRUCTOR_INVOKE,
   REDIRECTION,
   DIRECT_INVOKE,
+  DIRECT_USE,
 }
 
 /// Statically known use of an [Element].
@@ -311,6 +312,11 @@ class StaticUse {
     return new StaticUse.internal(element, StaticUseKind.GENERAL);
   }
 
+  /// Direct use of [element] as done with `--analyze-all` and `--analyze-main`.
+  factory StaticUse.directUse(Element element) {
+    return new StaticUse.internal(element, StaticUseKind.DIRECT_USE);
+  }
+
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! StaticUse) return false;