Include file path in JSON error when possible.

sdk/lib/_internal/pub/test/lock_file_test.dart

Index: sdk/lib/_internal/pub/test/lock_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/lock_file_test.dart b/sdk/lib/_internal/pub/test/lock_file_test.dart
index c6c5a229f96001042ca307027374fc3b940445bc..68d04ce07fba3adeb56306dfb701fabdba616466 100644
--- a/sdk/lib/_internal/pub/test/lock_file_test.dart
+++ b/sdk/lib/_internal/pub/test/lock_file_test.dart
@@ -110,6 +110,22 @@ packages:
         expect(lockFile.packages, isEmpty);
       });
 
+      test("throws if the top level is not a map", () {
+        expect(() {
+          new LockFile.parse('''
+not a map
+''', sources);
+        }, throwsFormatException);
+      });
+
+      test("throws if the contents of 'packages' is not a map", () {
+        expect(() {
+          new LockFile.parse('''
+packages: not a map
+''', sources);
+        }, throwsFormatException);
+      });
+
       test("throws if the version is missing", () {
         expect(() {
           new LockFile.parse('''