Add fast path to ListGetAsBytes for byte-size ExternalTypedData.
Add fast path to ListGetAsBytes for byte-size ExternalTypedData.

This speeds up deserializing snapshots which come in as ExternalUint8Array.

This API function should be generalized to be fast for all built-in array
types in a future CL.

R=ajohnsen@google.com, iposva@google.com

Review URL: https://codereview.chromium.org//300003006

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@36671 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk