Change List constructors.

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 e3418fedacd42bd8ea447b4c8511906c923b536f..b7b4075a98d5559cd70446ff761469ca29de8625 100644
--- a/sdk/lib/crypto/sha256.dart
+++ b/sdk/lib/crypto/sha256.dart
@@ -5,7 +5,7 @@
 // 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(64), super(16, 8, true) {
+  _SHA256() : _w = new List.fixedLength(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;