ngtsc needs to reflect over code to property compile it. It performs operations
such as enumerating decorators on a type, reading metadata from constructor
parameters, etc.
Depending on the format (ES5, ES6, etc) of the underlying code, the AST
structures over which this reflection takes place can be very different. For
example, in TS/ES6 code `class` declarations are `ts.ClassDeclaration` nodes,
but in ES5 code they've been downleveled to `ts.VariableDeclaration` nodes that
are initialized to IIFEs that build up the classes being defined.
The ReflectionHost abstraction allows ngtsc to perform these operations without
directly querying the AST. Different implementations of ReflectionHost allow
support for different code formats.
PR Close#24541
10da6a45c6