Make mappedBy lazy.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 8d87333d2e2a4052082a935caba2e2cacaa2a50d..89a550b7d6c7b595f0f4215fb26b28bc4dce66ae 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -67,10 +67,7 @@ class _ObjectArray<E> implements List<E> {
     Collections.forEach(this, f);
   }
 
-  Collection mappedBy(f(E element)) {
-    return Collections.mappedBy(
-        this, new _GrowableObjectArray.withCapacity(length), f);
-  }
+  Iterable mappedBy(f(E element)) => new MappedIterable(this, f);
 
   reduce(initialValue, combine(previousValue, E element)) {
     return Collections.reduce(this, initialValue, combine);
@@ -215,10 +212,7 @@ class _ImmutableArray<E> implements List<E> {
     Collections.forEach(this, f);
   }
 
-  Collection mappedBy(f(E element)) {
-    return Collections.mappedBy(
-        this, new _GrowableObjectArray.withCapacity(length), f);
-  }
+  Iterable mappedBy(f(E element)) => new MappedIterable(this, f);
 
   reduce(initialValue, combine(previousValue, E element)) {
     return Collections.reduce(this, initialValue, combine);