Dart is "a class-based optionally typed programming language for building web applications" and Google says that it's better suited for large-scale projects. "Developed with the goals of simplicity, efficiency, and scalability, the Dart language combines powerful new language features with familiar language constructs into a clear, readable syntax."
Dart is flexible because it's both static and dynamic, it's both for clients and servers and it's useful for both small scripts and large projects. Dart apps are easier to debug, to maintain and to develop collaboratively. The language is optimized for performance and doesn't allow programmers to use features like defining constants that have to be computed at runtime.
For now, no browser supports Dart, but it's likely that Chrome will address this problem in the near future. The code can be executed "either on a native virtual machine or on top of a JavaScript engine by using a compiler that translates Dart code to JavaScript." Google already provides a simple online IDE called Dartboard that lets you edit a small program using your browser, but Dartboard will evolve into a full-fledged online IDE.
Last year, a Google employee admitted that it will be difficult to convince developers and browser vendors to adopt the new language, but the fact that it's easy to convert Dart code to JavaScript is an important advantage. Google will promote it "as the language for serious web development on the web platform" and will "actively push for its standardization and adoption across the board". After all, "the goal of the Dart effort is ultimately to replace JavaScript as the lingua franca of web development on the open web platform." It's hard to say whether it will succeed, but it's worth trying to fix JavaScript's flaws by starting from the scratch. Google will have standardize the language, build development tools and develop powerful apps that use Dart to make a better case.
{ Thanks, Venkat. }