dartdevc source column is a little off for implicit-new constructors

[jmesserly]

after https://dart-review.googlesource.com/c/sdk/+/60402 lands, the expected stack trace column in
stacktrace_testfiles/throw_in_constructor looks like:

  var /*ddc.1:main*/ c = /*ddk.1:main*/ Class();

the DDK location looks fine but not the DDC location. What I think is going on: hover support conflicts with stack traces. The hover span for Class means we can't mark the start of Class in Dart corresponding to new lib.Class.new in JS. So we have to pick which one we prefer to work, or adjust our devtools hover implementation so they both work.