Make EfficientLength public, as EfficientLengthIterable.

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 329068244596f292d2c552e5d94d87d93ebb717f..14af73c69e42cee5f2898609fd3c964f1ef2ed40 100644
--- a/sdk/lib/collection/iterable.dart
+++ b/sdk/lib/collection/iterable.dart
@@ -90,7 +90,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
   Set<E> toSet() => new Set<E>.from(this);
 
   int get length {
-    assert(this is! EfficientLength);
+    assert(this is! EfficientLengthIterable);
     int count = 0;
     Iterator it = iterator;
     while (it.moveNext()) {
@@ -397,3 +397,34 @@ void _iterablePartsToStrings(Iterable iterable, List parts) {
   parts.add(penultimateString);
   parts.add(ultimateString);
 }
+
+
+/**
+ * Marker interface for [Iterable] implementations that have an efficient
+ * [length] getter.
+ *
+ * An iterable implementing this interface should have an "efficient"
+ * implementation of `length` that doesn't trigger iteration of the
+ * iterable. Being efficient doesn't necessarily require being constant
+ * time, but should at least attempt to have have execution time that is
+ * better than linear in the elements of the iterable.
+ *
+ * Methods that worry about reading the length of an `Iterable` because it
+ * may be inefficient or may trigger side-effects, or may even never complete,
+ * can first check if the iterable `is EfficientLengthIterableIterable`,
+ * and if so, use [length] without those concerns.
+ *
+ * The `EfficientLengthIterable` type should never be used as an API type
+ * assertion - neither as argument type or return type of a function.
+ * Always use [Iterable] for the type and just document the performance if it
+ * is relevant. This avoids needlessly restricting the values that can be used.
+ */
+abstract class EfficientLengthIterable<T> implements Iterable<T> {
+  /**
+   * Returns the number of elements in the iterable.
+   *
+   * This is an efficient operation that doesn't iterate through
+   * the elements.
+   */
+  int get length;
+}