fix #29228, let* precedence should match generated expression

pkg/dev_compiler/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_ast/js_ast.dart';
 import '../js_ast/precedence.dart';
 
 import 'js_names.dart' show TemporaryId;
 
@@ skipping 125 matching lines @@
     if (visitor is InterpolatedNodeAnalysis ||
         visitor is InstantiatorGeneratorVisitor) {
       variables.values.forEach((v) => v.accept(visitor));
       body.forEach((v) => v.accept(visitor));
     } else {
       toExpression().visitChildren(visitor);
     }
   }
 
   /// This generates as either a comma expression or a call.
-  int get precedenceLevel => variables.isEmpty ? EXPRESSION : CALL;
+  int get precedenceLevel => toExpression().precedenceLevel;
 
   /// Patch to pretend [Template] supports visitMetaLet.
   Instantiator _templateVisitMetaLet(InstantiatorGeneratorVisitor visitor) {
     var valueInstantiators = variables.values.map(visitor.visit);
     var bodyInstantiators = body.map(visitor.visit);
 
     return (args) => new MetaLet(
         new Map.fromIterables(
             variables.keys, valueInstantiators.map((i) => i(args))),
         bodyInstantiators.map((i) => i(args) as Expression).toList(),
@@ skipping 195 matching lines @@
     if (!_nestedFunction) hasYield = true;
     super.visitYield(node);
   }
 
   @override
   visitNode(Node node) {
     if (hasYield && hasThis) return; // found both, nothing more to do.
     super.visitNode(node);
   }
 }