anchor.diff:  
   /// An [UnresolvedSourceLocation] represents a location in the source
   // code which has not been precisely mapped to a token position.
  -class UnresolvedSourceLocation extends ServiceObject {
  +class UnresolvedSourceLocation extends ServiceObject implements Location {
     Script script;
     String scriptUri;
     int line;
  
positive.code:         params['column'] = col.toString();
        }
        Breakpoint bpt = await invokeRpc('addBreakpoint', params);
        if (bpt.resolved &&
            script.loaded &&
            script.tokenToLine(bpt.location.tokenPos) != line) {
          // TODO(turnidge): Can this still happen?
          script.getLine(line).possibleBpt = false;
        }
        return bpt;
      } on ServerRpcException catch(e) {
  
negative.code:     if (bpt.location.tokenPos != null) {
        line = tokenToLine(bpt.location.tokenPos);
      } else {
        UnresolvedSourceLocation loc = bpt.location;
        line = loc.line;
      }
      getLine(line).addBreakpoint(bpt);
    }
  