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..fa357977db9528d684a0e80048219341e800529c 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()) {
@@ -380,3 +382,17 @@ abstract class IterableBase<E> implements Iterable<E> {
     throw new RangeError.value(index);
   }
 }
+
+/**
+ * Marker interface for [Iterable] subclasses that have an efficient
+ * [length] implementation.
+ */
+abstract class EfficientLength {

[floitsch, 2013/10/10 13:04:53]

Move to _collection-dev.

+  /**
+   * Returns the number of elements in the iterable.
+   *
+   * This is an efficient operation that doesn't require iterating through
+   * the elements.
+   */
+  int get length;
+}