Generate correct intermediate indentation levels. Fixes #102.

If the expression nesting level jumps from, say, 3 to 6, it needs to consider all possible indentation permutations:

3:6
3:4:6
3:5:6
3:4:5:6

Before this fix, though, it would incorrectly do every subset starting at *zero*, not the minimum leading to tons of extra subsets to consider.

On my machine, this takes formatting dart2js from ~5s to 2.889s.

BUG=https://github.com/dart-lang/dart_style/issues/102
R=pquitslund@google.com

Review URL: https://chromiumcodereview.appspot.com//814913003