Migration tool rerun is slower than first run

[srawlins]

TL;DR We run InfoBuilder.explainMigration twice during a rerun, which I think builds up a huge graph of data. Additionally, it seems like even running explainMigration just the first time during a rerun takes much longer than the first time, like 3x to 8x as long.

As an anecdotal test, I deleted many test files from the analyzer code base, to reduce time, then ran dart migrate. I timed the following:

First run:

1. _FixCodeProcessor.runFirstPhase, (the NonNullableFix.prepareUnits), 22 seconds
2. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.processUnits), 22 seconds
3. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.finalizeUnits), 23 seconds
4. _FixCodeProcessor.runLaterPhases, (NonNullableFix.finish ,including InfoBuilder.explainMigration), 41 seconds

First "Rerun" in the UI:

1. _FixCodeProcessor.runFirstPhase, (the NonNullableFix.prepareUnits), 13 seconds
2. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.processUnits), 21 seconds
3. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.finalizeUnits), 28 seconds
4. _FixCodeProcessor.runLaterPhases, (NonNullableFix.finish ,including InfoBuilder.explainMigration), 146 seconds
5. NonNullableFix.rerun, (MigrationState.refresh, which also calls InfoBuilder.explainMigration), 200 seconds

Second "Rerun" in the UI:

1. _FixCodeProcessor.runFirstPhase, (the NonNullableFix.prepareUnits), 33 seconds
2. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.processUnits), 53 seconds
3. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.finalizeUnits), 42 seconds
4. _FixCodeProcessor.runLaterPhases, (NonNullableFix.finish ,including InfoBuilder.explainMigration), 330 seconds
5. NonNullableFix.rerun, (MigrationState.refresh, which also calls InfoBuilder.explainMigration), 387 seconds

Third "Rerun" in the UI:

1. _FixCodeProcessor.runFirstPhase, (the NonNullableFix.prepareUnits), 35 seconds
2. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.processUnits), 56 seconds
3. _FixCodeProcessor.runLaterPhases, (the NonNullableFix.finalizeUnits), 41 seconds
4. _FixCodeProcessor.runLaterPhases, (NonNullableFix.finish ,including InfoBuilder.explainMigration), 278 seconds
5. NonNullableFix.rerun, (MigrationState.refresh, which also calls InfoBuilder.explainMigration), 356 seconds