Don't run through iterable twice.

sdk/lib/_internal/compiler/implementation/js/nodes.dart

Index: sdk/lib/_internal/compiler/implementation/js/nodes.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js/nodes.dart b/sdk/lib/_internal/compiler/implementation/js/nodes.dart
index 3f753139a52a244459434ada2a8de21bc3a8864b..23ca6d09d315aceaf12c9e936c3557f37a2ad6b6 100644
--- a/sdk/lib/_internal/compiler/implementation/js/nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/js/nodes.dart
@@ -941,8 +941,10 @@ class ArrayInitializer extends Expression {
 
   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);