Update type signature of JSON codec.

Update type signature of JSON codec.

The encode function should definitely return a String.
The decode function returns an object of one of a known set of types.
We can't specify that, but we can use dynamic instead of Object.
This allows a user to write: JSON.decode('{"x":42}')["x"] without
warnings.

R=floitsch@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29661

[floitsch, 2013-10-30 17:57:40]

LGTM.

[Lasse Reichstein Nielsen, 2013-10-31 10:24:46]

Committed patchset #1 manually as r29661 (presubmit successful).