Cleanup of various DOM dart2js and dart_analyzer warnings

tools/dom/src/ImmutableListMixin.dart

Index: tools/dom/src/ImmutableListMixin.dart
diff --git a/tools/dom/src/ImmutableListMixin.dart b/tools/dom/src/ImmutableListMixin.dart
index df14b40ffe7584999b1d2876fdf33e6d87a7b1b7..2c05c11098b7fbb8dee9d051f8754a87d503ba06 100644
--- a/tools/dom/src/ImmutableListMixin.dart
+++ b/tools/dom/src/ImmutableListMixin.dart
@@ -47,7 +47,7 @@ abstract class ImmutableListMixin<E> implements List<E> {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
-  void remove(Object object) {
+  bool remove(Object object) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
@@ -59,7 +59,7 @@ abstract class ImmutableListMixin<E> implements List<E> {
     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.");
   }