Whether Einstein's 1905 constant-speed-of-light postulate is true or false depends on whether light pulses bunch up in front of the moving source or not:
http://www.einstein-online.info/imag...ler_static.gif
http://www.einstein-online.info/imag...ource_blue.gif
Light pulses don't bunch up - bunching up obviously violates the principle of relativity. The speed of light VARIES with the speed of the light source, as shown by the Michelson-Morley experiment in 1887:
John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE."
http://philsci-archive.pitt.edu/1743/2/Norton.pdf
Pentcho Valev