We have some changes coming down the pipe that should reduce memory usage; in the meantime, you can increase the maximim heap of the editor by editing the DartEditor.ini file. The line you want to change is the -Xmx line.
You can find the DartEditor.ini file in the same directory as the executable. On the Mac, you have to right click the application and choose 'Show Package Contents'. The ini file is a few directories down in Contents/MacOS.
Our next integration builds (and the current continuous build) will pick up these changes. Please re-open this if you still see the problem!