dissallow -release mode on emulators (#4204)

* dissallow -release mode on emulators

* have the help text print the default mode

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

diff --git a/packages/flutter_tools/lib/src/base/utils.dart b/packages/flutter_tools/lib/src/base/utils.dart
index 312739a..052e671 100644
--- a/packages/flutter_tools/lib/src/base/utils.dart
+++ b/packages/flutter_tools/lib/src/base/utils.dart
@@ -39,6 +39,12 @@
   return str;
 }
 
+String toTitleCase(String str) {
+  if (str.isEmpty)
+    return str;
+  return str.substring(0, 1).toUpperCase() + str.substring(1);
+}
+
 /// Return the plural of the given word (`cat(s)`).
 String pluralize(String word, int count) => count == 1 ? word : word + 's';