[macros] Add host implementation notes.

working/macros/host-notes.md

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+# Macro Host Notes
+
+Notes about macro host (analyzer, CFE) implementations.
+
+Some related discussion
+[on issue 55784](https://github.com/dart-lang/sdk/issues/55784).
+
+## Analyzer
+
+1.  Build library cycles. Use imports and exports to form library cycles. Sort
+    them topologically.
+2.  With all dependency library cycles loaded (which means that we can ask them
+    for elements with all types ready), take the next library cycle.
+3.  Parse all user-written Dart files, create `LibraryBuilder` objects.
+4.  Build `Element`s for all declarations - classes, methods, etc. Just
+    elements, types are not known yet.
+5.  Create the `LibraryMacroApplier` instance, fill `LibraryBuilder`s with
+    `_MacroApplication`s. The order of the macro applications is defined by the
+    specification.
+6.  Run the types phase. Iterate over `LibraryBuilder`s, and run the types phase
+    for macro applications. We get Dart code as output. Add library
+    augmentations from these Dart code strings. Build elements for these library
+    augmentations, just like (4). Look for more macro applications, just like
+    (5).
+7.  Build export scopes for `LibraryBuilder`s. This includes any user-written
+    declarations, handling re-exports.
+8.  Resolve `TypeAnnotation`s in all `LibraryBuilder`s.
+9.  Run the declarations phase. Like (6), but not only build elements, and find
+    new macro applications, but also resolve type annotations like (8). If new
+    declarations are added, rebuild export scopes.
+10. Run many other element linking steps - build synthetic constructors, enum
+    constants, resolve constants, resolve metadata, perform type inference, etc.
+11. Run the definitions phase. Like (9), still produce new library
+    augmentations.
+12. Dispose all macro applications, so free resources in the remote isolate.
+13. Merge all macro results into single Dart code. Discard all transitory
+    library augmentations, add the new, final one. Do not create new elements,
+    not like (6) or (9), instead merge `ClassElement`s, update offsets, etc.
+    These elements have everything ready in them - types, resolution for
+    constants, etc. We don’t want to redo this work. And we cannot create new
+    elements - there are already references to these elements in other parts of
+    the element model.
+
+## CFE
+
+TODO