Cleanup of various DOM dart2js and dart_analyzer warnings

tools/dom/src/ImmutableListMixin.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 dart.dom.html;
 
 abstract class ImmutableListMixin<E> implements List<E> {
   // From Iterable<$E>:
   Iterator<E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
@@ skipping 29 matching lines @@
   }
 
   E removeAt(int pos) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   E removeLast() {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
-  void remove(Object object) {
+  bool remove(Object object) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   void removeWhere(bool test(E element)) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   void retainWhere(bool test(E element)) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
-  void setRange(int start, int end, Iterable<E> iterable, [int skipCount]) {
+  void setRange(int start, int end, Iterable<E> iterable, [int skipCount = 0]) {
     throw new UnsupportedError("Cannot setRange on immutable List.");
   }
 
   void removeRange(int start, int end) {
     throw new UnsupportedError("Cannot removeRange on immutable List.");
   }
 
   void replaceRange(int start, int end, Iterable<E> iterable) {
     throw new UnsupportedError("Cannot modify an immutable List.");
   }
 
   void fillRange(int start, int end, [E fillValue]) {
     throw new UnsupportedError("Cannot modify an immutable List.");
   }
 }