Missing onAmbiguousSupertypes callback from DDC and Analyzer

[askeksa-google]

The fixes in #31996 and #31984 will work for stand-alone Fasta and dart2js, but since DDC and Analyzer do not pass an onAmbiguousSupertypes callback, the class hierarchy computation will throw an exception when an ambiguous supertype is encountered.

[peter-ahe-google]

I don't understand why DDC needs its own ClassHierarchy. I hope we can help them move away from that, potentially by using the ClassHierarchy that Fasta already computed.