Guard calls to ByteAddr when a ByteArray copy length is 0.

Guard calls to ByteAddr when a ByteArray copy length is 0.

It is permitted to specify a start index equal to the array length
when the copy length is 0.  This runs afoul of an assertion in
ByteAddr that validates 0 <= index < length.  Consistent with other
methods to retrieve a raw element address, we now guard this call with
an implicit check for this boundary condition by ensuring length is
greater than zero.

In addition, some simple unit tests for ByteArray::Copy have been added.

BUG=1891
Committed: https://code.google.com/p/dart/source/detail?r=4772

[turnidge, 2012-03-01 00:12:29]

lgtm