Documenting --all option.

BUG=
R=rnystrom@google.com

Review URL: https://codereview.appspot.com/77460043
Documenting new args for pub serve.

BUG=
R=nweiz@google.com

Review URL: https://codereview.appspot.com/77500044
Moving the dart-vm page from Dart Up and Running to the tools area.

BUG=
R=kathyw@google.com

Review URL: https://codereview.appspot.com/78460043
Updated benchmarking page.

Added information about output of the benchmarking_harness and fixed image scaling
Merge pull request #319 from caffinatedmonkey/benchmarking

Updated benchmarking page.
Adding info on the dependency_overrides option for pubspec.

BUG=
R=kevmoo@google.com, rnystrom@google.com

Review URL: https://codereview.appspot.com/78820044