Don't run through iterable twice.

sdk/lib/_internal/compiler/implementation/js/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;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 923 matching lines @@
 }
 
 class ArrayInitializer extends Expression {
   final int length;
   // We represent the array as sparse list of elements. Each element knows its
   // position in the array.
   final List<ArrayElement> elements;
 
   ArrayInitializer(this.length, this.elements);
 
-  factory ArrayInitializer.from(Iterable<Expression> expressions) =>
-      new ArrayInitializer(expressions.length, _convert(expressions));
+  factory ArrayInitializer.from(Iterable<Expression> expressions) {
+    List<ArrayElement> elements = _convert(expressions);
+    return new ArrayInitializer(elements.length, elements);
+  }
 
   accept(NodeVisitor visitor) => visitor.visitArrayInitializer(this);
 
   void visitChildren(NodeVisitor visitor) {
     for (ArrayElement element in elements) element.accept(visitor);
   }
 
   ArrayInitializer _clone() => new ArrayInitializer(length, elements);
 
   int get precedenceLevel => PRIMARY;
@@ skipping 156 matching lines @@
 class Comment extends Statement {
   final String comment;
 
   Comment(this.comment);
 
   accept(NodeVisitor visitor) => visitor.visitComment(this);
   Comment _clone() => new Comment(comment);
 
   void visitChildren(NodeVisitor visitor) {}
 }