[et] Allow disabling the "cute" output mode, so folks can `grep` the logs about flutter HOT 7 CLOSED

Oh man, well tools like ag will change their output format when they detect they are being piped to another process. git uses the flag --porcelain to indicate output that is verbose and meant to be parsed by another tool.

I think making --verbose imply that is fine and I don't recommend those other approaches.

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Ah, I just filed #147936 which is a duplicate. We need to check that stdout is a terminal type and disable the pretty printing if its not. It is also ok to allow manually disabling pretty printing to terminal types too.

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As an aside, we need to go a bit out our way to fool Xcode since it pretends to be a terminal but can't handle being a terminal quite right. GN does this detection.

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https://api.dart.dev/stable/3.3.4/dart-io/Stdout/hasTerminal.html may be what we need (and the one for stderr).

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Update: We do some checks for hasTerminal and supportsAnsiEscapes, but it sounds like those are either insufficient, or give false-positives.

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When I do:

flutter % et build -v | grep supportsAnsiEscapes
supportsAnsiEscapes: false | hasTerminal: false

... it appears detection does work, so I'm assuming we're not guarding something we should be.

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The fix is as simple as:

diff --git a/tools/engine_tool/lib/src/logger.dart b/tools/engine_tool/lib/src/logger.dart
index 28bd847885..8b4565d94b 100644
--- a/tools/engine_tool/lib/src/logger.dart
+++ b/tools/engine_tool/lib/src/logger.dart
@@ -169,6 +169,8 @@ class Logger {
   /// and emits a carriage return.
   void clearLine() {
     if (!io.stdout.hasTerminal || _test) {
+      // Just emit a newline in tests or when not connected to a terminal.
+      _ioSinkWrite(io.stdout, '\n');
       return;
     }

However, it is next to impossible to test because the Logger does a bunch of things with global state.

I'm going to provide a bit more TLC to logger.dart and then land the fix.

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