Change List constructors.

sdk/lib/crypto/hmac.dart

Index: sdk/lib/crypto/hmac.dart
diff --git a/sdk/lib/crypto/hmac.dart b/sdk/lib/crypto/hmac.dart
index 6d0d502cb1221fb4ad70cb0be58bac6bfe623aca..4a48d76ff8362ae6f33525e146b2c7a3a46b3af2 100644
--- a/sdk/lib/crypto/hmac.dart
+++ b/sdk/lib/crypto/hmac.dart
@@ -21,7 +21,7 @@ class _HMAC implements HMAC {
 
     // Zero-pad the key until its size is equal to the block size of the hash.
     if (_key.length < blockSize) {
-      var newKey = new List(blockSize);
+      var newKey = new List.fixedLength(blockSize);
       newKey.setRange(0, _key.length, _key);
       for (var i = _key.length; i < blockSize; i++) {
         newKey[i] = 0;
@@ -30,7 +30,7 @@ class _HMAC implements HMAC {
     }
 
     // Compute inner padding.
-    var padding = new List(blockSize);
+    var padding = new List.fixedLength(blockSize);
     for (var i = 0; i < blockSize; i++) {
       padding[i] = 0x36 ^ _key[i];
     }