dart2js cps: Remove dart2dart from cps pipeline and clean up.

pkg/compiler/lib/src/tree_ir/optimization/optimization.dart

 library tree_ir.optimization;
 
 import '../tree_ir_nodes.dart';
-import '../../constants/values.dart' as values;
-import 'variable_merger.dart';
 
+export 'statement_rewriter.dart' show StatementRewriter;
 export 'variable_merger.dart' show VariableMerger;
+export 'loop_rewriter.dart' show LoopRewriter;
+export 'logical_rewriter.dart' show LogicalRewriter;
 export 'pull_into_initializers.dart' show PullIntoInitializers;
 
-part 'logical_rewriter.dart';
-part 'loop_rewriter.dart';
-part 'statement_rewriter.dart';
-
 /// An optimization pass over the Tree IR.
 abstract class Pass {
   /// Applies optimizations to root, rewriting it in the process.
-  void rewrite(RootNode root);
+  void rewrite(FunctionDefinition root);
 
   String get passName;
 }