plugin.needs to assert dependencies

lib/api.js

@@ -141,6 +141,16 @@ function Api (plugins, defaultConfig) {
       }
 
       const name = plugin.name
+
+      if (plugin.needs) {
+        for (const needed of plugin.needs) {
+          const found = create.plugins.some((p) => p.name === needed)
+          if (!found) {
+            throw new Error(`plugin "${name ?? '?'}" needs plugin "${needed}" but not found`)
+          }
+        }
+      }
+
       if (plugin.manifest) {
         create.manifest = u.merge.manifest(
           create.manifest,

test/api.js

@@ -140,6 +140,39 @@ tape('plugin cannot be named global', function (t) {
   t.end()
 })
 
+tape('plugin needs another plugin', function (t) {
+  // core, not a plugin.
+  var Create = Api([{
+    manifest: {},
+    init: function (api) {
+      return {}
+    }
+  }])
+
+  t.throws(() => {
+    Create.use({
+      name: 'x',
+      needs: ['y'],
+      init: function () { }
+    })
+  }, 'throws on missing plugin')
+
+  Create.use({
+    name: 'foo',
+    init: function () { }
+  })
+
+  t.doesNotThrow(() => {
+    Create.use({
+      name: 'bar',
+      needs: ['foo'],
+      init: function () { }
+    })
+  }, 'does not throw on existing plugin')
+
+  t.end()
+})
+
 tape('compound (array) plugins', function (t) {
   // core, not a plugin.
   var Create = Api([{