Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/use_unused_api.dart

Index: pkg/compiler/lib/src/use_unused_api.dart
diff --git a/pkg/compiler/lib/src/use_unused_api.dart b/pkg/compiler/lib/src/use_unused_api.dart
index 772e231d42147936dc3fa53afcc37da5487a5737..ed69c6f273f6c5ba1107b33681c6e20f3bf5aaab 100644
--- a/pkg/compiler/lib/src/use_unused_api.dart
+++ b/pkg/compiler/lib/src/use_unused_api.dart
@@ -40,7 +40,7 @@ import 'resolution/semantic_visitor.dart' as semantic_visitor;
 import 'resolution/operators.dart' as operators;
 import 'script.dart';
 import 'source_file_provider.dart' as source_file_provider;
-import 'ssa/ssa.dart' as ssa;
+import 'ssa/nodes.dart' as ssa;
 import 'tree/tree.dart' as tree;
 import 'util/util.dart' as util;
 import 'world.dart';
@@ -343,4 +343,4 @@ useTreeVisitors() {
 
 useDeferred([deferred.DeferredLoadTask task]) {
   task.dump();
-}
+}