fix(ivy): don't crash on an unknown localref target (#33454)

Previously the template binder would crash when encountering an unknown
localref (# reference) such as `<div #ref="foo">` when no directive has
`exportAs: "foo"`.

With this commit, the compiler instead generates a template diagnostic error
informing the user about the invalid reference.

PR Close #33454

packages/compiler/src/render3/view/t2_binder.ts

@@ -260,16 +260,16 @@ class DirectiveBinder<DirectiveT extends DirectiveMeta> implements Visitor {
         // This could be a reference to a component if there is one.
         dirTarget = directives.find(dir => dir.isComponent) || null;
       } else {
-        // This is a reference to a directive exported via exportAs. One should exist.
+        // This should be a reference to a directive exported via exportAs.
         dirTarget =
             directives.find(
                 dir => dir.exportAs !== null && dir.exportAs.some(value => value === ref.value)) ||
             null;
-
-        // Check if a matching directive was found, and error if it wasn't.
+        // Check if a matching directive was found.
         if (dirTarget === null) {
-          // TODO(alxhub): Return an error value here that can be used for template validation.
-          throw new Error(`Assertion error: failed to find directive with exportAs: ${ref.value}`);
+          // No matching directive was found - this reference points to an unknown target. Leave it
+          // unmapped.
+          return;
         }
       }