Let list-classes call Collections.joinList (which avoids allocating an iterator).

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index f5e8ec33ffe34cd8158c294d7a3511cf839261b6..fb9f3582c61b1bc7f7f0cb05f09381cc0bb0924e 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -70,7 +70,7 @@ class _ObjectArray<E> implements List<E> {
   }
 
   String join([String separator]) {
-    return Collections.join(this, separator);
+    return Collections.joinList(this, separator);
   }
 
   List mappedBy(f(E element)) {
@@ -278,7 +278,7 @@ class _ImmutableArray<E> implements List<E> {
   }
 
   String join([String separator]) {
-    return Collections.join(this, separator);
+    return Collections.joinList(this, separator);
   }
 
   reduce(initialValue, combine(previousValue, E element)) {