Separate the control structure expression from its statement DRT-W1041

From the style guide for the flutter repo:

DO separate the control structure expression from its statement.

Don't put the statement part of an if, for, while, do on the same line as the expression, even if it is short. Doing so makes it unclear that there is relevant code there. This is especially important for early returns.

BAD:

if (notReady) return;

if (notReady)
  return;
else print('ok')

while (condition) i += 1;

GOOD:

if (notReady)
  return;

if (notReady)
  return;
else
  print('ok')

while (condition)
  i += 1;

Note that this rule can conflict with the Dart formatter, and should not be enabled when the Dart formatter is used.