Remove js.ArrayElement.

pkg/compiler/lib/src/js/printer.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;
 
 class Printer extends Indentation implements NodeVisitor {
   final bool shouldCompressOutput;
   leg.Compiler compiler;
   leg.CodeBuffer outBuffer;
@@ skipping 756 matching lines @@
     }
     out(node.value);
   }
 
   visitLiteralNull(LiteralNull node) {
     out("null");
   }
 
   visitArrayInitializer(ArrayInitializer node) {
     out("[");
-    List<ArrayElement> 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 {
+    List<Expression> elements = node.elements;
+    for (int i = 0; i < elements.length; i++) {
+      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)
+        // and [1,,] (array with 1 and a hole) would become [1,] = [1].
         out(",");
+        continue;
       }
+      if (i != 0) spaceOut();
+      visitNestedExpression(element, ASSIGNMENT,
+                            newInForInit: false, newAtStatementBegin: false);
+      // We can skip the trailing "," for the last element (since it's not
+      // an array hole).
+      if (i != elements.length - 1) out(",");
     }
     out("]");
   }
 
-  visitArrayElement(ArrayElement node) {
+  visitArrayHole(ArrayHole node) {
     throw "Unreachable";
   }
 
   visitObjectInitializer(ObjectInitializer node) {
     // Print all the properties on one line until we see a function-valued
     // property.  Ideally, we would use a proper pretty-printer to make the
     // decision based on layout.
     List<Property> properties = node.properties;
     out("{");
     indentMore();
@@ skipping 359 matching lines @@
         codes.add(nthLetter((n ~/ nameSpaceSize) % LETTERS));
       }
       codes.add(charCodes.$0 + digit);
       newName = new String.fromCharCodes(codes);
     }
     assert(new RegExp(r'[a-zA-Z][a-zA-Z0-9]*').hasMatch(newName));
     maps.last[oldName] = newName;
     return newName;
   }
 }