_JenkinsSmiHash should be exposed by SDK

[stereotype441]

The _JenkinsSmiHash class (sdk/lib/math/jenkins_smi_hash.dart) is useful enough that it has now been duplicated in 6 places:

pkg/analysis_server/lib/src/protocol/protocol_internal.dart (*)
pkg/analyzer/lib/src/generated/utilities_general.dart
runtime/observatory/lib/object_graph.dart
sdk/lib/html/dart2js/html_dart2js.dart
sdk/lib/html/dart2js/html_dartium.dart
tools/dom/templates/html/impl/imtl_ClientRect.darttemplate

(*Note that the implementation in analysis_server is could be replaced with a reference to the one in analyzer).

We should expose the SDK's implementation of _JenkinsSmiHash so that all the duplicate definitions can be eliminated.

This would also allow us to synthesize better code when generating default implementations of hashCode (an IntelliJ feature @jwren is currently working on).

[floitschG]

Duplicate of #11617.