fix #27320, better DDC temp generation

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

Index: pkg/dev_compiler/lib/src/js_ast/nodes.dart
diff --git a/pkg/dev_compiler/lib/src/js_ast/nodes.dart b/pkg/dev_compiler/lib/src/js_ast/nodes.dart
index 354e4f69115119211e029b41d5fc04cc0216e2b4..8317fdbeeaebb766f1c29d738902e6889b515de4 100644
--- a/pkg/dev_compiler/lib/src/js_ast/nodes.dart
+++ b/pkg/dev_compiler/lib/src/js_ast/nodes.dart
@@ -709,11 +709,16 @@ abstract class Expression extends Node {
 
   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();