Revert "Library functions can never be constructors."

sdk/lib/_internal/lib/core_patch.dart

Index: sdk/lib/_internal/lib/core_patch.dart
diff --git a/sdk/lib/_internal/lib/core_patch.dart b/sdk/lib/_internal/lib/core_patch.dart
index 6d829fa0fcb01d8e89ebb5f7bdb60072a7cb4fcb..b6f538ffec9700b8d3e709384b5d42b256ae2fce 100644
--- a/sdk/lib/_internal/lib/core_patch.dart
+++ b/sdk/lib/_internal/lib/core_patch.dart
@@ -228,18 +228,19 @@ class Stopwatch {
   static int _now() => Primitives.timerTicks();
 }
 
+class _ListConstructorSentinel extends JSInt {
+  const _ListConstructorSentinel();
+}
+
 // Patch for List implementation.
 @patch
 class List<E> {
   @patch
-  factory List([int length]) = List._;
-
-  factory List._([length = const ListConstructorSentinel()]) {
-    if (length == const ListConstructorSentinel()) {
+  factory List([int length = const _ListConstructorSentinel()]) {
+    if (length == const _ListConstructorSentinel()) {
       return new JSArray<E>.emptyGrowable();
     }
-    int typeCheckedLength = length;
-    return new JSArray<E>.fixed(typeCheckedLength);
+    return new JSArray<E>.fixed(length);
   }
 
   @patch