[dart2js] Report error when input file is not .dart or .dill

Bug: #61361
Change-Id: I499c4c3818804b48fdb043860d4c7aabb4ffd6db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446685
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Mayank Patke <[email protected]>

Author: fishythefish

pkg/compiler/lib/src/dart2js.dart

@@ -674,10 +674,14 @@ Future<api.CompilationResult> compile(
     if (sourceOrDill.endsWith('.dart')) {
       options.add('${Flags.entryUri}=$file');
       entryUri = file;
-    } else {
-      assert(sourceOrDill.endsWith('.dill'));
+    } else if (sourceOrDill.endsWith('.dill')) {
       options.add('${Flags.inputDill}=$file');
       inputDillUri = file;
+    } else {
+      _fail(
+        "Unexpected file '$sourceOrDill'.\n"
+        "Only .dart and .dill extensions are supported.",
+      );
     }
   }