begin method
Called when the beginning of a tag is encountered.
Implementation
@override
void begin(TagContext tc, String data) {
if (data.isEmpty)
tc.error("The $name tag requires a condition");
String beg, end;
if (data.startsWith('(') && data.endsWith(')')) {
beg = end = "";
} else {
beg = needsVar_ ? "(var ": "(";
end = ")";
}
tc.writeln("\n${tc.pre}$name $beg$data$end {${tc.getLineNumberComment()}");
tc.indent();
}