Implement modular compilation

lib/src/compiler/js_metalet.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(jmesserly): import from its own package
-import '../js/js_ast.dart';
-import '../js/precedence.dart';
+import '../js_ast/js_ast.dart';
+import '../js_ast/precedence.dart';
 
 import 'js_names.dart' show TemporaryId;
 
 /// A synthetic `let*` node, similar to that found in Scheme.
 ///
 /// For example, postfix increment can be desugared as:
 ///
 ///     // psuedocode mix of Scheme and JS:
 ///     (let* (x1=expr1, x2=expr2, t=x1[x2]) { x1[x2] = t + 1; t })
 ///
@@ skipping 299 matching lines @@
   @override
   visitYield(Yield node) {
     if (!_nestedFunction) hasYield = true;
   }
 
   @override
   visitNode(Node node) {
     if (!hasYield) super.visitNode(node);
   }
 }