Make Iterable.toList more efficient if the length is known.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 300b2efcc4cf6a77b23136738fdb9ac988f96427..523449c25ecdb451400015d50c3c44ec397f045f 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -336,8 +336,30 @@ abstract class Iterable<E> {
    * The elements are in iteration order.
    * The list is fixed-length if [growable] is false.
    */
-  List<E> toList({ bool growable: true }) =>
-      new List<E>.from(this, growable: growable);
+  List<E> toList({ bool growable: true }) {
+    if (this is! EfficientLength) {

[floitsch, 2016/05/20 12:46:02]

This check is already done in List.from.

+      return new List<E>.from(this, growable: growable);
+    }
+    return _toListKnownLength(this.length, growable);
+  }
+
+  /// Converts this Iterable to a list with the given length.
+  List<E> _toListKnownLength(int length, bool growable) {
+    List<E> result;
+    if (growable) {
+      result = new List<E>()..length = length;
+    } else {
+      result = new List<E>(length);
+    }
+    int i = 0;
+    for (var element in this) {
+      result[i++] = element;
+    }
+    if (i != length) {
+      throw new ConcurrentModificationError(this);
+    }
+    return result;
+  }
 
   /**
    * Creates a [Set] containing the same elements as this iterable.