startMultilineMode method
Starts multiline mode with the given type.
Throws InvalidMultilineModeException if already in multiline mode.
Implementation
void startMultilineMode(MultilineMode mode) {
if (multilineMode != MultilineMode.none) {
throw InvalidMultilineModeException(
currentMode: multilineMode.name,
attemptedMethod: 'start${_modeMethodName(mode)}',
);
}
multilineMode = mode;
}