[testing] Make the FLUTTER_STORAGE_BASE_URL warning non-fatal (#128335)

Presubmit testing and CI testing of Flutter using a custom storage location for engine artifacts must be able to use the --fatal-warnings flag without failing due to the custom artifact location.

This change adds an option that makes this warning non-fatal. The new --no-fatal-storage-url-warning flag makes the --fatal-warnings flag ignore the warning that a custom artifact download URL is being used by setting the environment variable FLUTTER_STORAGE_BASE_URL.

Bug: #127683

- [X ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [X ] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
- [X ] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [X ] I signed the [CLA].
- [X ] I listed at least one issue that this PR fixes in the description above.
- [X ] I updated/added relevant documentation (doc comments with `///`).

packages/flutter_tools/lib/src/base/logger.dart

@@ -46,7 +46,8 @@ abstract class Logger {
   /// since the last time it was set to false.
   bool hadErrorOutput = false;
 
-  /// If true, then [printWarning] has been called at least once for this logger
+  /// If true, then [printWarning] has been called at least once with its
+  /// "fatal" argument true for this logger
   /// since the last time it was reset to false.
   bool hadWarningOutput = false;
 
@@ -124,6 +125,7 @@ abstract class Logger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   });
 
   /// Display normal output of the command. This should be used for things like
@@ -314,6 +316,7 @@ class DelegatingLogger implements Logger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   }) {
     _delegate.printWarning(
       message,
@@ -322,6 +325,7 @@ class DelegatingLogger implements Logger {
       indent: indent,
       hangingIndent: hangingIndent,
       wrap: wrap,
+      fatal: fatal,
     );
   }
 
@@ -481,8 +485,9 @@ class StdoutLogger extends Logger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   }) {
-    hadWarningOutput = true;
+    hadWarningOutput = hadWarningOutput || fatal;
     _status?.pause();
     message = wrapText(message,
       indent: indent,
@@ -820,8 +825,9 @@ class BufferLogger extends Logger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   }) {
-    hadWarningOutput = true;
+    hadWarningOutput = hadWarningOutput || fatal;
     _warning.writeln(terminal.color(
       wrapText(message,
         indent: indent,
@@ -965,6 +971,7 @@ class VerboseLogger extends DelegatingLogger {
         int? indent,
         int? hangingIndent,
         bool? wrap,
+        bool fatal = true,
       }) {
     hadWarningOutput = true;
     _emit(

packages/flutter_tools/lib/src/cache.dart

@@ -278,6 +278,9 @@ class Cache {
   // Whether to cache the unsigned mac binaries. Defaults to caching the signed binaries.
   bool useUnsignedMacBinaries = false;
 
+  // Whether the warning printed when a custom artifact URL is used is fatal.
+  bool fatalStorageWarning = true;
+
   static RandomAccessFile? _lock;
   static bool _lockEnabled = true;
 
@@ -340,12 +343,11 @@ class Cache {
           // version --machine"). It's not really a "warning" though, so print it
           // in grey. Also, make sure that it isn't counted as a warning for
           // Logger.warningsAreFatal.
-          final bool oldWarnings = _logger.hadWarningOutput;
           _logger.printWarning(
             'Waiting for another flutter command to release the startup lock...',
             color: TerminalColor.grey,
+            fatal: false,
           );
-          _logger.hadWarningOutput = oldWarnings;
           printed = true;
         }
         await Future<void>.delayed(const Duration(milliseconds: 50));
@@ -522,6 +524,7 @@ class Cache {
     _logger.printWarning(
       'Flutter assets will be downloaded from $overrideUrl. Make sure you trust this source!',
       emphasis: true,
+      fatal: false,
     );
     _hasWarnedAboutStorageOverride = true;
   }

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -1250,6 +1250,7 @@ class NotifyingLogger extends DelegatingLogger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   }) {
     _sendMessage(LogMessage('warning', message));
   }

packages/flutter_tools/test/commands.shard/hermetic/attach_test.dart

@@ -1138,8 +1138,9 @@ class StreamLogger extends Logger {
     int? indent,
     int? hangingIndent,
     bool? wrap,
+    bool fatal = true,
   }) {
-    hadWarningOutput = true;
+    hadWarningOutput = hadWarningOutput || fatal;
     _log('[stderr] $message');
   }