Add OptId{Class,Function}Declaration for ExportDefaultDeclaration

Closes estree#98. This implementation was initially suggested by Sebastian McKenzie.

es2015.md

@@ -4,6 +4,7 @@ This document specifies the extensions to the [core ESTree AST types](es5.md) to
 
 - [Programs](#programs)
 - [Functions](#functions)
+  - [OptIdFunctionDeclaration](#optidfunctiondeclaration)
 - [Statements](#statements)
   - [ForOfStatement](#forofstatement)
 - [Declarations](#declarations)
@@ -26,6 +27,7 @@ This document specifies the extensions to the [core ESTree AST types](es5.md) to
   - [ClassDeclaration](#classdeclaration)
   - [ClassExpression](#classexpression)
   - [MetaProperty](#metaproperty)
+  - [OptIdClassDeclaration](#optidclassdeclaration)
 - [Modules](#modules)
   - [ModuleDeclaration](#moduledeclaration)
   - [ModuleSpecifier](#modulespecifier)
@@ -61,6 +63,13 @@ extend interface Function {
 }
 ```
 
+## OptIdFunctionDeclaration
+
+```js
+interface OptIdFunctionDeclaration <: FunctionDeclaration {
+    id: Identifier | null;
+}
+
 # Statements
 
 ## ForOfStatement
@@ -299,6 +308,14 @@ interface MetaProperty <: Expression {
 }
 ```
 
+## OptIdClassDeclaration
+
+```js
+interface OptIdClassDeclaration <: ClassDeclaration {
+    id: Identifier | null;
+}
+```
+
 # Modules
 
 ## ModuleDeclaration
@@ -397,7 +414,7 @@ An exported variable binding, e.g., `{foo}` in `export {foo}` or `{bar as foo}`
 ```js
 interface ExportDefaultDeclaration <: ModuleDeclaration {
     type: "ExportDefaultDeclaration";
-    declaration: Declaration | Expression;
+    declaration: OptIdFunctionDeclaration | OptIdClassDeclaration | Expression;
 }
 ```