fix #27320, better DDC temp generation

pkg/dev_compiler/lib/src/js_ast/nodes.dart

 // Copyright (c) 2012, 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.
 
 part of js_ast;
 
 abstract class NodeVisitor<T> implements TypeRefVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 691 matching lines @@
     for (var node in exprs) {
       comma = (comma == null) ? node : new Binary(op, comma, node);
     }
     return comma;
   }
 
   int get precedenceLevel;
 
   Statement toStatement() => new ExpressionStatement(toVoidExpression());
   Statement toReturn() => new Return(this);
+
+  // TODO(jmesserly): make this return a Yield?
   Statement toYieldStatement({bool star: false}) =>
       new ExpressionStatement(new Yield(this, star: star));
 
   Expression toVoidExpression() => this;
-  Expression toAssignExpression(Expression left) => new Assignment(left, this);
+  Expression toAssignExpression(Expression left, [String op]) =>
+      new Assignment.compound(left, op, this);
+
+  // TODO(jmesserly): make this work for more cases?
   Statement toVariableDeclaration(Identifier name) =>
       new VariableDeclarationList(
           'let', [new VariableInitialization(name, this)]).toStatement();
 }
 
 class LiteralExpression extends Expression {
   final String template;
   final List<Expression> inputs;
 
   LiteralExpression(this.template) : inputs = const [];
@@ skipping 1176 matching lines @@
   final List<ModuleItem> body;
   Module(this.body, {this.name});
 
   accept(NodeVisitor visitor) => visitor.visitModule(this);
   void visitChildren(NodeVisitor visitor) {
     for (ModuleItem item in body) item.accept(visitor);
   }
 
   Module _clone() => new Module(body);
 }