Add common interface for cryptographic hash functions to lib/crypto.dart.

tests/lib/src/crypto/sha1Test.dart

Index: tests/lib/src/crypto/sha1Test.dart
diff --git a/tests/lib/src/crypto/sha1Test.dart b/tests/lib/src/crypto/sha1Test.dart
index 9d23fd8999cfb9be0755d8375485fc198cfd9cbf..7128cd262d5f4b5408e4beb530e77562a0ff23f0 100644
--- a/tests/lib/src/crypto/sha1Test.dart
+++ b/tests/lib/src/crypto/sha1Test.dart
@@ -538,7 +538,7 @@ void testStatic() {
   ];
   for (var i = 0; i < expected_values.length; i++) {
     Expect.equals(expected_values[i],
-                  digestToString(SHA1.digest(createTestArr(i))));
+                  digestToString(new SHA1().update(createTestArr(i)).digest()));
   }
 }