Throw an error when generating different results for the same file

This error was already thrown when different results were generated
due to different entrypoints, but not for the same entrypoint. In
practice I don't think this can come up with the module migrator,
though, due to #99.

lib/src/migration_visitor.dart

@@ -6,6 +6,10 @@
 
 import 'dart:collection';
 
+import 'package:meta/meta.dart';
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+
 // The sass package's API is not necessarily stable. It is being imported with
 // the Sass team's explicit knowledge and approval. See
 // https://github.com/sass/dart-sass/issues/236.
@@ -14,9 +18,7 @@ import 'package:sass/src/importer.dart';
 import 'package:sass/src/import_cache.dart';
 import 'package:sass/src/visitor/recursive_ast.dart';
 
-import 'package:meta/meta.dart';
-import 'package:source_span/source_span.dart';
-
+import 'exception.dart';
 import 'patch.dart';
 
 /// A visitor that migrates a stylesheet.
@@ -87,10 +89,20 @@ abstract class MigrationVisitor extends RecursiveAstVisitor {
     _patches = [];
     _currentUrl = node.span.sourceUrl;
     super.visitStylesheet(node);
+
     var results = getMigratedContents();
     if (results != null) {
-      _migrated[node.span.sourceUrl] = results;
+      var existingResults = _migrated[_currentUrl];
+      if (existingResults != null && existingResults != results) {
+        throw MigrationException(
+            "The migrator has found multiple possible migrations for "
+            "${p.prettyUri(_currentUrl)}, depending on the context in which "
+            "it's loaded.");
+      }
+
+      _migrated[_currentUrl] = results;
     }
+
     _patches = oldPatches;
     _currentUrl = oldUrl;
   }

lib/src/migrator.dart

@@ -87,7 +87,8 @@ abstract class Migrator extends Command<Map<Uri, String>> {
         if (allMigrated.containsKey(file) &&
             migrated[file] != allMigrated[file]) {
           throw MigrationException(
-              "$file is migrated in more than one way by these entrypoints.");
+              "The migrator has found multiple possible migrations for $file, "
+              "depending on the context in which it's loaded.");
         }
         allMigrated[file] = migrated[file];
       }