Reapply "Make Map constructors return LinkedHashMap."

sdk/lib/collection/iterable.dart

Index: sdk/lib/collection/iterable.dart
diff --git a/sdk/lib/collection/iterable.dart b/sdk/lib/collection/iterable.dart
index 6dbf92ddf7926368a5cf3073b72d5c8974b72e06..9cbdf897caabe9d6e9ba04664887b3856f1e4660 100644
--- a/sdk/lib/collection/iterable.dart
+++ b/sdk/lib/collection/iterable.dart
@@ -54,7 +54,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
     return true;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     Iterator<E> iterator = this.iterator;
     if (!iterator.moveNext()) return "";
     StringBuffer buffer = new StringBuffer();
@@ -245,7 +245,7 @@ abstract class IterableBase<E> implements Iterable<E> {
     return true;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     Iterator<E> iterator = this.iterator;
     if (!iterator.moveNext()) return "";
     StringBuffer buffer = new StringBuffer();