Suppress 'Info: Compiling without sound null safety' message when building flutter_tools (#74740)

The CFE now logs to stdout by default when compiling a program which has
non-null-safe dependencies. Since flutter_tools has not yet migrated, we
need to suppress this message when compiling the tool.

Fixes https://github.com/flutter/flutter/issues/74366

bin/internal/shared.bat

@@ -156,9 +156,9 @@ GOTO :after_subroutine
     POPD
 
     IF "%FLUTTER_TOOL_ARGS%" == "" (
-      "%dart%" --snapshot="%snapshot_path%" --packages="%flutter_tools_dir%\.packages" --no-enable-mirrors "%script_path%"
+      "%dart%" --verbosity=error --snapshot="%snapshot_path%" --packages="%flutter_tools_dir%\.packages" --no-enable-mirrors "%script_path%"
     ) else (
-      "%dart%" "%FLUTTER_TOOL_ARGS%" --snapshot="%snapshot_path%" --packages="%flutter_tools_dir%\.packages" "%script_path%"
+      "%dart%" "%FLUTTER_TOOL_ARGS%" --verbosity=error --snapshot="%snapshot_path%" --packages="%flutter_tools_dir%\.packages" "%script_path%"
     )
     IF "%ERRORLEVEL%" NEQ "0" (
       ECHO Error: Unable to create dart snapshot for flutter tool. 1>&2

bin/internal/shared.sh

@@ -151,7 +151,7 @@ function upgrade_flutter () (
 
     retry_upgrade
 
-    "$DART" --disable-dart-dev $FLUTTER_TOOL_ARGS --snapshot="$SNAPSHOT_PATH" --packages="$FLUTTER_TOOLS_DIR/.packages" --no-enable-mirrors "$SCRIPT_PATH"
+    "$DART" --verbosity=error --disable-dart-dev $FLUTTER_TOOL_ARGS --snapshot="$SNAPSHOT_PATH" --packages="$FLUTTER_TOOLS_DIR/.packages" --no-enable-mirrors "$SCRIPT_PATH"
     echo "$revision" > "$STAMP_PATH"
   fi
   # The exit here is extraneous since the function is run in a subshell, but