Remove js.ArrayElement.

pkg/compiler/lib/src/js/printer.dart

Index: pkg/compiler/lib/src/js/printer.dart
diff --git a/pkg/compiler/lib/src/js/printer.dart b/pkg/compiler/lib/src/js/printer.dart
index f64bdfb0840752ab5bb41694424df10571d14492..0c0223d02b781fec4517db6ac9e0364957a405a6 100644
--- a/pkg/compiler/lib/src/js/printer.dart
+++ b/pkg/compiler/lib/src/js/printer.dart
@@ -774,29 +774,24 @@ class Printer extends Indentation implements NodeVisitor {
 
   visitArrayInitializer(ArrayInitializer node) {
     out("[");
-    List<ArrayElement> elements = node.elements;
+    List<Expression> elements = node.elements;
     int elementIndex = 0;
-    for (int i = 0; i < node.length; i++) {
-      if (elementIndex < elements.length &&
-          elements[elementIndex].index == i) {
-        visitNestedExpression(elements[elementIndex].value, ASSIGNMENT,
-                              newInForInit: false, newAtStatementBegin: false);
-        elementIndex++;
-        // We can avoid a trailing "," if there was an element just before. So
-        // `[1]` and `[1,]` are the same, but `[,]` and `[]` are not.
-        if (i != node.length - 1) {
-          out(",");
-          spaceOut();
-        }
-      } else {
+    for (int i = 0; i < elements.length; i++) {
+      visitNestedExpression(elements[i], ASSIGNMENT,
+                            newInForInit: false, newAtStatementBegin: false);
+      // We can avoid a trailing "," if there was an element just before. So
+      // `[1]` and `[1,]` are the same, but `[,]` and `[]` are not.
+      if (i != elements.length - 1 ||
+          (i == 0 && elements[i] is ArrayHole)) {

[floitsch, 2014/11/28 12:42:50]

Why the check for "i == 0" ?

I think it should be:

for (int i = 0 ... ) {
  Expression element = elements[i];
  if (element is ArrayHole) {
    // Note that array holes must have a trailing "," even if they are
    // in last position. Otherwise `[,]` (having length 1) would become
    // equal to `[]` (the empty array).
    out(",");
    continue;
  }
  if (i != 0) spaceOut();
  visitNestedExpression(entry, ...);
  // We can skip the trailing "," for the last element (since it's not
  // an array hole).
  if (i != node.length - 1) out(",");
}

[sigurdm, 2014/11/28 14:52:11]

Yes, something was wrong in my logic. Thanks.
Adding a test for the array-hole case.

         out(",");
+        spaceOut();

[floitsch, 2014/11/28 12:42:50]

This means that we now have "[, ]" instead of "[,]".

[sigurdm, 2014/11/28 14:52:11]

True
Fixed

       }
     }
     out("]");
   }
 
-  visitArrayElement(ArrayElement node) {
-    throw "Unreachable";
+  visitArrayHole(ArrayHole node) {

[floitsch, 2014/11/28 12:42:50]

Should be unreachable again, if you use my code.

[sigurdm, 2014/11/28 14:52:11]

Done.

+    // An array hole is represented by the empty string.
   }
 
   visitObjectInitializer(ObjectInitializer node) {