Assignment expressions in tree IR.

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 'variable_merger.dart' show VariableMerger;
+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);
 
   String get passName;
 }