The results of the toibe index are heavily influenced by google's search.
People working on Dart working at google would likely be familiar with how to create a page that has improved google visibility compared to say... the R or objective C communities. This isn't a "ohh, nefarious!" type thing but rather a "how to make a web page that conforms to everything that google search would want" is part of the way one would think.
It is possible to game tiobe given that its strongly influenced by google's search results. If you go glance at delphi's history in 2008, you'll see gamification of it - http://web.archive.org/web/20120312021442/http://delphi.org/... http://web.archive.org/web/20120312021416/http://delphi.org/... http://web.archive.org/web/20120312021435/http://delphi.org/... - and then you go look at the graph for delphi, and you'll see http://www.tiobe.com/tiobe-index/delphi-object-pascal/ .
Simply having well crafted pages that get indexed well for Dart would be sufficient to bump a language on Tiobe. And that's why its all a load of marketing.