Configure the dev server for exploded dart apps.

mojoconfig

 # This is a configuration file for devtools (`mojo_run`, `mojo_test) running
 # within a Mojo checkout.
 
 # The content has to parse to a Python dictionary literal. Strings of the form
 # '@{ABC}' are aliases that will be substituted for their values before
 # evaluation:
 #  '@{BUILD_DIR}': path to the output directory
 
 {
   # Each dev server will be configured as specified and mapped for the
   # indicated host using --map-origin.
   'dev_servers': [
     {
       'host': 'https://core.mojoapps.io/',
       # First matching prefix will apply. Within the directiories specified for
       # a prefix, first location that contains the requested path will apply.
       'mappings': [
         ('packages/', ['@{BUILD_DIR}/gen/dart-pkg/packages']),
-        ('', ['@{BUILD_DIR}']),
+        ('', [
+          # We map two directiories, so that both exploded dart apps under
+          # checkout root and built apps in the build directory are available.
+          # For example, one could refer to the apps of either type using urls:
+          #  - https://core.mojoapps.io/spinning_cube.mojo
+          #  - https://core.mojoapps.io/examples/dart/device_info/main.dart
+          '@{BUILD_DIR}',
+          '.'
+        ]),
       ],
     },
   ],
 }