Make List.remove return boolean.

sdk/lib/_collection_dev/list.dart

Index: sdk/lib/_collection_dev/list.dart
diff --git a/sdk/lib/_collection_dev/list.dart b/sdk/lib/_collection_dev/list.dart
index 93e49aeff356f8d9fbe4dd27957ec96df9d790ef..c14cb502a6be53e582ff8bfeda6da28424a32bc3 100644
--- a/sdk/lib/_collection_dev/list.dart
+++ b/sdk/lib/_collection_dev/list.dart
@@ -35,7 +35,7 @@ abstract class FixedLengthListMixin<E>  {
         "Cannot add to a fixed-length list");
   }
 
-  void remove(E element) {
+  bool remove(E element) {
     throw new UnsupportedError(
         "Cannot remove from a fixed-length list");
   }
@@ -112,7 +112,7 @@ abstract class UnmodifiableListMixin<E> {
 
   void add(E value) {
     throw new UnsupportedError(
-        "Cannot add to an unmodifiable list");
+      "Cannot add to an unmodifiable list");
   }
 
   E insert(int index, E value) {
@@ -130,7 +130,7 @@ abstract class UnmodifiableListMixin<E> {
         "Cannot add to an unmodifiable list");
   }
 
-  void remove(E element) {
+  bool remove(E element) {
     throw new UnsupportedError(
         "Cannot remove from an unmodifiable list");
   }