Catch assignments to read-only properties and indexers

[Chuck]

This is an error that Cobra misses that is dropping through to C#.

class X

	get two as int
		return 2

	
	def main is shared
		x = X()
		x.two = 4  # .error. cannot be assigned to

# drops through to C#:
# Property or indexer "X.Two" cannot be assigned to -- it is read only (C#)