Include file path in JSON error when possible.

sdk/lib/_internal/pub/lib/src/exceptions.dart

Index: sdk/lib/_internal/pub/lib/src/exceptions.dart
diff --git a/sdk/lib/_internal/pub/lib/src/exceptions.dart b/sdk/lib/_internal/pub/lib/src/exceptions.dart
index 74af539763f4e7a9b7306ec00384c177d56dd157..fc83c0c3c538b75c0ab472fc7c93bee298605535 100644
--- a/sdk/lib/_internal/pub/lib/src/exceptions.dart
+++ b/sdk/lib/_internal/pub/lib/src/exceptions.dart
@@ -26,6 +26,19 @@ class ApplicationException implements Exception {
   String toString() => message;
 }
 
+/// An exception class for exceptions that are intended to be seen by the user
+/// and are associated with a problem in a file at some path.
+class FileException implements ApplicationException {
+  final String message;
+
+  /// The path to the file that was missing or erroneous.
+  final String path;
+
+  FileException(this.message, this.path);
+
+  String toString() => message;
+}
+
 /// A class for exceptions that wrap other exceptions.
 class WrappedException extends ApplicationException {
   /// The underlying exception that [this] is wrapping, if any.