[ Widget Preview ] Refactor @Preview() detection and code generation (flutter#168307)

This change makes the detection of `@Preview(...)` annotations more
robust by converting the analyzer AST directly to generated code rather
than simply copying and pasting the original source. This allows for
proper handling of the following cases WRT import prefixing:

  - The use of `const`
  - String interpolation
  - Other uses of constant variables in non-trivial contexts
  - Etc.

Author: bkonyi