Use module.export(getters, true) for const ExportNamedDeclaration.

These changes let the runtime know that the values returned by the getter
functions for

  export const a = 1, b = 2;

will never change. Unfortunately, it's more difficult to detect that the
same is true for

  const a = 1, b = 2;
  export { a, b };

though that's an area for future improvement.

Part of #134.

lib/import-export-visitor.js

@@ -26,14 +26,15 @@ class ImportExportVisitor extends Visitor {
         codeToInsert += '"use strict";';
       }
 
-      const namedExports = toModuleExport(
-        this,
-        bodyInfo.hoistedExportsMap
-      );
+      const addExportsMap = (map, constant) => {
+        const namedExports = toModuleExport(this, map, constant);
+        if (namedExports) {
+          codeToInsert += namedExports;
+        }
+      };
 
-      if (namedExports) {
-        codeToInsert += namedExports;
-      }
+      addExportsMap(bodyInfo.hoistedExportsMap, false);
+      addExportsMap(bodyInfo.hoistedConstExportsMap, true);
 
       if (bodyInfo.hoistedExportsString) {
         codeToInsert += bodyInfo.hoistedExportsString;
@@ -303,7 +304,14 @@ class ImportExportVisitor extends Visitor {
       }
 
       hoistExports(this, path, specifierMap, "declaration");
-      addExportedLocalNames(this, specifierMap);
+
+      if (! isConstExportDeclaration(decl)) {
+        // We can skip adding declared names to this.exportedLocalNames if
+        // the declaration was a const-kinded VariableDeclaration, because
+        // the assignmentVisitor will not need to worry about changes to
+        // these variables.
+        addExportedLocalNames(this, specifierMap);
+      }
 
       return;
     }
@@ -528,6 +536,7 @@ function getBlockBodyInfo(visitor, path) {
     bodyInfo.insertCharIndex = insertCharIndex;
     bodyInfo.insertNodeIndex = insertNodeIndex;
     bodyInfo.hoistedExportsMap = Object.create(null);
+    bodyInfo.hoistedConstExportsMap = Object.create(null);
     bodyInfo.hoistedExportsString = "";
     bodyInfo.hoistedImportsString = "";
 
@@ -575,6 +584,7 @@ function hoistExports(visitor, exportDeclPath, mapOrString, childName) {
   }
 
   const bodyInfo = getBlockBodyInfo(visitor, exportDeclPath);
+  const constant = isConstExportDeclaration(exportDeclPath.getValue());
 
   if (typeof mapOrString !== "string") {
     const keys = Object.keys(mapOrString);
@@ -587,7 +597,9 @@ function hoistExports(visitor, exportDeclPath, mapOrString, childName) {
 
       for (let j = 0; j < localCount; ++j) {
         addToSpecifierMap(
-          bodyInfo.hoistedExportsMap,
+          constant
+            ? bodyInfo.hoistedConstExportsMap
+            : bodyInfo.hoistedExportsMap,
           exported,
           locals[j]
         );
@@ -601,6 +613,25 @@ function hoistExports(visitor, exportDeclPath, mapOrString, childName) {
   visitor.madeChanges = true;
 }
 
+function isConstExportDeclaration(exportDecl) {
+  if (exportDecl) {
+    if (exportDecl.type === "ExportDefaultDeclaration") {
+      const dd = exportDecl.declaration;
+      return (dd.type !== "FunctionDeclaration" &&
+              dd.type !== "ClassDeclaration");
+    }
+
+    if (exportDecl.type === "ExportNamedDeclaration") {
+      const dd = exportDecl.declaration;
+      return !! dd &&
+        dd.type === "VariableDeclaration" &&
+        dd.kind === "const";
+    }
+  }
+
+  return false;
+}
+
 function makeUniqueKey(visitor) {
   return visitor.nextKey++;
 }
@@ -802,7 +833,7 @@ function toModuleImport(source, specifierMap, uniqueKey) {
   return code;
 }
 
-function toModuleExport(visitor, specifierMap) {
+function toModuleExport(visitor, specifierMap, constant) {
   let code = "";
   const exportedKeys = Object.keys(specifierMap);
   const keyCount = exportedKeys.length;
@@ -834,7 +865,9 @@ function toModuleExport(visitor, specifierMap) {
     }
   }
 
-  code += "});";
+  // The second argument to module.export indicates whether the getter
+  // functions provided in the first argument are constant or not.
+  code += constant ? "},true);" : "});";
 
   return code;
 }

test/output/declarations-basic/expected.js

@@ -1,4 +1,4 @@
-"use strict";module.export({a:()=>a,b:()=>b,c:()=>c,d:()=>d});const a = 1;
+"use strict";module.export({c:()=>c,d:()=>d});module.export({a:()=>a,b:()=>b},true);const a = 1;
 const b = function () {
   return d;
 };

test/output/name/expected.js

@@ -1,4 +1,4 @@
-"use strict";module.export({id:()=>id,name:()=>name});const path = require("path");
+"use strict";module.export({id:()=>id,name:()=>name},true);const path = require("path");
 
 const id = module.id,
   name = path.basename(__filename);