Change new List(n) to return fixed length list.

sdk/lib/crypto/sha256.dart

Index: sdk/lib/crypto/sha256.dart
diff --git a/sdk/lib/crypto/sha256.dart b/sdk/lib/crypto/sha256.dart
index 84dabf9bb0341022ad5c591b141c4b8f54b5777b..3df0ffd0627b4c09c4ffb6b05534ea453b6d5069 100644
--- a/sdk/lib/crypto/sha256.dart
+++ b/sdk/lib/crypto/sha256.dart
@@ -7,7 +7,7 @@ part of dart.crypto;
 // The SHA256 hasher is used to compute an SHA256 message digest.
 class _SHA256 extends _HashBase implements SHA256 {
   // Construct a SHA256 hasher object.
-  _SHA256() : _w = new List.fixedLength(64), super(16, 8, true) {
+  _SHA256() : _w = new List(64), super(16, 8, true) {
     // Initial value of the hash parts. First 32 bits of the fractional parts
     // of the square roots of the first 8 prime numbers.
     _h[0] = 0x6a09e667;