Rename all proposal plugins to -proposal- from -transform- (#6570)

packages/babel-plugin-proposal-nullish-coalescing-operator/src/index.js

@@ -0,0 +1,41 @@
+import syntaxNullishCoalescingOperator from "@babel/plugin-syntax-nullish-coalescing-operator";
+
+export default function({ types: t }, { loose = false }) {
+  return {
+    inherits: syntaxNullishCoalescingOperator,
+
+    visitor: {
+      LogicalExpression(path) {
+        const { node, scope } = path;
+        if (node.operator !== "??") {
+          return;
+        }
+
+        const ref = scope.generateUidIdentifierBasedOnNode(node.left);
+        scope.push({ id: ref });
+
+        const assignment = t.assignmentExpression("=", t.clone(ref), node.left);
+
+        path.replaceWith(
+          t.conditionalExpression(
+            // We cannot use `!= null` in spec mode because
+            // `document.all == null` and `document.all` is not "nullish".
+            loose
+              ? t.binaryExpression("!=", assignment, t.nullLiteral())
+              : t.logicalExpression(
+                  "&&",
+                  t.binaryExpression("!==", assignment, t.nullLiteral()),
+                  t.binaryExpression(
+                    "!==",
+                    t.clone(ref),
+                    scope.buildUndefinedNode(),
+                  ),
+                ),
+            t.clone(ref),
+            node.right,
+          ),
+        );
+      },
+    },
+  };
+}