Remove the crypto factories

pkg/crypto/lib/src/hmac.dart

Index: pkg/crypto/lib/src/hmac.dart
diff --git a/pkg/crypto/lib/src/hmac.dart b/pkg/crypto/lib/src/hmac.dart
index 2d07cff9a152502d1ecbd755c6c8fea4ab24a095..59c177109497d52a6319baa2a60f81765d628a0b 100644
--- a/pkg/crypto/lib/src/hmac.dart
+++ b/pkg/crypto/lib/src/hmac.dart
@@ -4,16 +4,32 @@
 
 part of crypto;
 
-class _HMAC implements HMAC {
+/**
+ * Hash-based Message Authentication Code support.
+ *
+ * The [add] method is used to add data to the message. The [digest] and
+ * [close] methods are used to extract the message authentication code.
+ */
+// TODO(floitsch): make Hash implement Sink, EventSink or similar.
+class HMAC {
   bool _isClosed = false;
 
-  _HMAC(Hash this._hash, List<int> this._key) : _message = [];
+  /**
+   * Create an [HMAC] object from a [Hash] and a key.
+   */
+  HMAC(Hash this._hash, List<int> this._key) : _message = [];
 
+  /**
+   * Add a list of bytes to the message.
+   */
   add(List<int> data) {
     if (_isClosed) throw new StateError("HMAC is closed");
     _message.addAll(data);
   }
 
+  /**
+   * Extract the message digest as a list of bytes without closing [this].
+   */
   List<int> get digest {
     var blockSize = _hash.blockSize;
 
@@ -58,11 +74,25 @@ class _HMAC implements HMAC {
     return _hash.close();
   }
 
+  /**
+   * Perform the actual computation and extract the message digest
+   * as a list of bytes.
+   */
   List<int> close() {
     _isClosed = true;
     return digest;
   }
 
+  /**
+   * Verify that the HMAC computed for the data so far matches the
+   * given message digest.
+   *
+   * This method should be used instead of memcmp-style comparisons
+   * to avoid leaking information via timing.
+   *
+   * Throws an exception if the given digest does not have the same
+   * size as the digest computed by this HMAC instance.
+   */
   bool verify(List<int> digest) {
     var computedDigest = this.digest;
     if (digest.length != computedDigest.length) {