flow-tool: cleanup

Change-Id: Ib561f65dc03c555cfbf1dbe3519759b9b4f12e32
flow: fixes a crash on abnormal parser termination

Change-Id: I997278c22291a23397ce67d4a2758a12d69ee0d7
flow: adds positional ParamList::replace()

Change-Id: Ib4c868becfce1ba23fbb4e6f88a4ed22dcc64929
flow: auto-complete positional params with default args if possible, too

Change-Id: I311b3782f90492c783ac0f6b09ae5283a6404a2e