Make SourceSpanException.source a getter
Make SourceSpanException.source a getter

The override in

https://github.com/dart-lang/string_scanner/blob/master/lib/src/exception.dart

is causing a problem in DDC / strong mode.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1722603005 .

dart-lang/source_span