Add EfficientLength marker interface to some iterabels.

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 9cbdf897caabe9d6e9ba04664887b3856f1e4660..0e5989d28d82cd2293c5753e8a010a2e5cbd10db 100644
--- a/sdk/lib/collection/iterable.dart
+++ b/sdk/lib/collection/iterable.dart
@@ -85,6 +85,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
   Set<E> toSet() => new Set<E>.from(this);
 
   int get length {
+    assert(this is! EfficientLength);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
@@ -276,6 +277,7 @@ abstract class IterableBase<E> implements Iterable<E> {
   Set<E> toSet() => new Set<E>.from(this);
 
   int get length {
+    assert(this is! EfficientLength);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {