Update hashCode to hash the entire message

Update hashCode to hash the entire message

This is inspired by the way the hash code is generated for protocol
buffers in Java, see:

https://code.google.com/p/protobuf/source/browse/trunk/java/src/main/java/com/google/protobuf/AbstractMessage.java#188

For lists the Jenkins hash function is used.

Tests are in https://chromiumcodereview.appspot.com/63173006.

R=sigmund@google.com
BUG=

Committed: https://github.com/dart-lang/protobuf/commit/c5fdfcb

[Siggi Cherem (dart-lang), 2013-11-07 21:06:34]

lgtm

[Søren Gjesse, 2013-11-08 07:58:19]

Committed patchset #2 manually as rc5fdfcb (presubmit successful).