Let SyncStartIterable extend IterableBase instead of Iterable.

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 02ef267082e332bf5b6d1e49159a20c8eecb49e0..596a982fc5d677d1af2bcb6cbb0cc1bea887610c 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -4085,7 +4085,7 @@ class SyncStarIterator implements Iterator {
 /// An Iterable corresponding to a sync* method.
 ///
 /// Each invocation of a sync* method will return a new instance of this class.
-class SyncStarIterable extends Iterable {
+class SyncStarIterable extends IterableBase {
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //