Not sure what you mean: MS has a history of abandoning its developer platforms and frameworks that thier developers have invested in, so much so that there's no longer a clear UI story for building native desktop windows apps, i.e. VB6, Silverlight, WinForms and WPF are all effectively deprecated. Although they have long support life, when they officially abandon a platform they also refuse to Open Source it so others can keep it alive (e.g. VB6 http://bit.ly/KPMUS6). I personally can't see how anyone can put trust into building native Windows desktop apps (i.e. over web apps) given the repeated a history of abandonment.

Even the #1 feature for VS.NET is to continue developing very popular XNA framework:
http://visualstudio.uservoice.com/forums/121579-visual-studi...

Likewise on the server side there's been history of deprecated frameworks a lot of developers have invested a lot of energy into learning, e.g: .asmx, CSF, WCF, WCF/REST, WSE, WCF DataServices, RIA

Whilst VisualStudio is a great IDE, I find it a subpar experience without R#. The major advantages Dart has over C# is that it still provides an enjoyable experience to develop even without an IDE which also includes support for the most popular text editors: https://www.dartlang.org/tools/
If you like IDE and tooling support, the DartEditor offers a good experience with built-in analyzer, debugging and refactoring support you can expect from a well engineered language.

By contrast, C#'s configuration model, msbuild project format, heavy frameworks and tooling makes it unfeasible to develop without an IDE.

The other major advantage Dart has is that it compiles down to JS where even the compiler is completely self-hosting and runs inside a browser without plugins: http://try.dartlang.org/
Being able to share the same code on client and server and having a single integrated development full-stack experience is a huge win in re-usability and utility.

Another killer feature is that the language and tooling is cross-platform which supports Windows, OSX and Linux. Something .NET devs often miss out on is the value and utility of being able to host apps on cost-effective Linux servers.

Google continues to invest a tonne of resources in Dart and Polymer which are massive undertakings that are providing a much simplified and consistent experience for developing large, complex web apps. Nothing like Dart or Polymer exists. Dart is a platform that transpiles to JS, includes a native Dart VM, an entire toolchain including IDE, analyzer and debugger both in Dart Editor as well as in Chrome, in both the Dart VM as well as debugging with Source maps.

The worlds best VM engineers work on the Dart VM, i.e. the same pedigree responsible for the StrongTalk VM that was later acquired by Sun to form the basis of the world-class Java Hotspot VM that later went on to develop V8, are now leading the development on the Dart VM.

The excellence shows itself in the consistency and minimalism goals in the language, providing a productive, iterative dynamic language experience for fast prototyping with the benefit of static analysis with optional typing when scaling up to a maintainable, well-documented code-base (best of both worlds).

Not only is Dart a productive dynamic language, it also has excellent performance, the best performance of all languages I benchmarked with a port of Peter Norvig's Sudoku solver: https://github.com/dartist/sudoku_solver#benchmarks