Add ForeignByteBuffer which exposes the foreign memory

[karlklose]

Most of our SDK relies on the getForeign method of the implementation class of ByteBuffer, so we have to type variables with 'var' to avoid warnings.

We should all a class ForeignByteBuffer that extends the interface of ByteBuffer with the getForeign method.

[mezoni]

Please did not forget about that the Dart VM has a limitations in the list length. Eg. in 32-bit mode maximum length of list is a maximum SMI value (32 bit - 1 bit = 31 bit). This means that it can not cover the entire address space (only the half of all possible address space).

[mit-mit]

CC @DmitryOlshansky-google