Warn instead of crashing for bad URLs

lib/src/migration_visitor.dart

@@ -13,6 +13,7 @@ import 'package:sass/src/ast/sass.dart';
 import 'package:sass/src/visitor/recursive_ast.dart';
 
 import 'package:meta/meta.dart';
+import 'package:source_span/source_span.dart';
 
 import 'patch.dart';
 import 'utils.dart';
@@ -66,10 +67,16 @@ abstract class MigrationVisitor extends RecursiveAstVisitor {
   }
 
   /// Visits the stylesheet at [dependency], resolved relative to [source].
+  ///
+  /// This returns true if the dependency is successfully visited and false
+  /// otherwise.
   @protected
-  void visitDependency(Uri dependency, Uri source) {
-    var stylesheet = parseStylesheet(source.resolveUri(dependency));
+  bool visitDependency(Uri dependency, Uri source, {FileSpan context}) {
+    var stylesheet =
+        parseStylesheet(source.resolveUri(dependency), context: context);
+    if (stylesheet == null) return false;
     visitStylesheet(stylesheet);
+    return true;
   }
 
   /// Returns the migrated contents of this file, or null if the file does not
@@ -96,7 +103,8 @@ abstract class MigrationVisitor extends RecursiveAstVisitor {
     if (migrateDependencies) {
       for (var import in node.imports) {
         if (import is DynamicImport) {
-          visitDependency(Uri.parse(import.url), node.span.sourceUrl);
+          visitDependency(Uri.parse(import.url), node.span.sourceUrl,
+              context: import.span);
         }
       }
     }
@@ -108,7 +116,7 @@ abstract class MigrationVisitor extends RecursiveAstVisitor {
   visitUseRule(UseRule node) {
     super.visitUseRule(node);
     if (migrateDependencies) {
-      visitDependency(node.url, node.span.sourceUrl);
+      visitDependency(node.url, node.span.sourceUrl, context: node.span);
     }
   }
 }

lib/src/migrator.dart

@@ -45,7 +45,9 @@ abstract class Migrator extends Command<Map<Uri, String>> {
   Map<Uri, String> run() {
     var allMigrated = Map<Uri, String>();
     for (var entrypoint in argResults.rest) {
-      var migrated = migrateFile(canonicalize(Uri.parse(entrypoint)));
+      var canonicalUrl = canonicalize(Uri.parse(entrypoint));
+      if (canonicalUrl == null) continue;
+      var migrated = migrateFile(canonicalUrl);
       for (var file in migrated.keys) {
         if (allMigrated.containsKey(file) &&
             migrated[file] != allMigrated[file]) {

lib/src/migrators/division.dart

@@ -199,7 +199,8 @@ class _DivisionMigrationVisitor extends MigrationVisitor {
           expectsNumericResult: true);
       return true;
     } else {
-      emitWarning("Could not determine whether this is division", node.span);
+      emitWarning("Could not determine whether this is division",
+          context: node.span);
       super.visitBinaryOperationExpression(node);
       return false;
     }

lib/src/migrators/module.dart

@@ -153,7 +153,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
       if (nameArgument is! StringExpression ||
           (nameArgument as StringExpression).text.asPlain == null) {
         emitWarning("get-function call may require \$module parameter",
-            nameArgument.span);
+            context: nameArgument.span);
         return;
       }
       var fnName = nameArgument as StringExpression;
@@ -206,7 +206,8 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
               existingArgName: _findArgNameSpan(arg));
           name = 'adjust';
         } else {
-          emitWarning("Could not migrate malformed '$name' call", node.span);
+          emitWarning("Could not migrate malformed '$name' call",
+              context: node.span);
           return;
         }
       }
@@ -270,7 +271,10 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
 
     var oldConfiguredVariables = _configuredVariables;
     _configuredVariables = Set();
-    visitDependency(Uri.parse(import.url), _currentUrl);
+    if (!visitDependency(Uri.parse(import.url), _currentUrl,
+        context: import.span)) {
+      return;
+    }
     _namespaces[_lastUrl] = namespaceForPath(import.url);
 
     // Pass the variables that were configured by the importing file to `with`,

lib/src/runner.dart

@@ -78,7 +78,7 @@ class MigratorRunner extends CommandRunner<Map<Uri, String>> {
       for (var url in migrated.keys) {
         assert(url.scheme == null || url.scheme == "file",
             "$url is not a file path.");
-        if (argResults['verbose']) print("Mirating ${p.prettyUri(url)}");
+        if (argResults['verbose']) print("Migrating ${p.prettyUri(url)}");
         File(url.toFilePath()).writeAsStringSync(migrated[url]);
       }
     }

lib/src/utils.dart

@@ -4,6 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+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
@@ -21,11 +22,19 @@ import 'patch.dart';
 final _filesystemImporter = FilesystemImporter('.');
 
 /// Returns the canonical version of [url].
-Uri canonicalize(Uri url) => _filesystemImporter.canonicalize(url);
+Uri canonicalize(Uri url, {FileSpan context}) {
+  var canonicalUrl = url == null ? null : _filesystemImporter.canonicalize(url);
+  if (canonicalUrl == null) {
+    emitWarning("Could not find Sass file at '${p.prettyUri(url)}'.",
+        context: context);
+  }
+  return canonicalUrl;
+}
 
 /// Parses the file at [url] into a stylesheet.
-Stylesheet parseStylesheet(Uri url) {
-  var canonicalUrl = _filesystemImporter.canonicalize(url);
+Stylesheet parseStylesheet(Uri url, {FileSpan context}) {
+  var canonicalUrl = canonicalize(url, context: context);
+  if (canonicalUrl == null) return null;
   var result = _filesystemImporter.load(canonicalUrl);
   return Stylesheet.parse(result.contents, result.syntax, url: canonicalUrl);
 }
@@ -58,9 +67,13 @@ Patch patchDelete(FileSpan span, {int start = 0, int end}) {
       span.file.span(span.start.offset + start, span.start.offset + end), "");
 }
 
-/// Emits a warning with [message] and [context];
-void emitWarning(String message, FileSpan context) {
-  print(context.message("WARNING - $message"));
+/// Emits a warning with [message] and optionally [context];
+void emitWarning(String message, {FileSpan context}) {
+  if (context == null) {
+    print("WARNING - $message");
+  } else {
+    print(context.message("WARNING - $message"));
+  }
 }
 
 /// An exception thrown by a migrator.

test/migrators/division/bad_paths.hrx

@@ -0,0 +1,13 @@
+<==> arguments
+--migrate-deps
+
+<==> input/entrypoint.scss
+@import "does_not_exist";
+
+<==> log.txt
+line 1, column 9 of entrypoint.scss: WARNING - Could not find Sass file at 'does_not_exist'.
+  ,
+1 | @import "does_not_exist";
+  |         ^^^^^^^^^^^^^^^^
+  '
+Nothing to migrate!