Improve yield support and restrict usage of strict

- Partially reverting 91cdb93e57 and eaf3911c31 and reimplement
- Add generators support for objects and classes
- Only classes can have static methods so restrict use of it

Special thanks to @rvanvelzen and @kzc for reviewing this patch and
providing constructive feedback over and over again.

lib/ast.js

@@ -487,8 +487,9 @@ var AST_Arrow = DEFNODE("Arrow", null, {
     $documentation: "An ES6 Arrow function ((a) => b)"
 }, AST_Lambda);
 
-var AST_ConciseMethod = DEFNODE("ConciseMethod", "static", {
+var AST_ConciseMethod = DEFNODE("ConciseMethod", "is_generator static", {
     $propdoc: {
+        is_generator: "is generatorFn or not",
         static: "[boolean] whether this method is static (classes only)",
     },
     $documentation: "An ES6 concise method inside an object or class"
@@ -1241,6 +1242,21 @@ var AST_True = DEFNODE("True", null, {
     value: true
 }, AST_Boolean);
 
+/* -----[ Yield ]----- */
+
+var AST_Yield = DEFNODE("Yield", "expression is_star", {
+    $documentation: "A `yield` statement",
+    $propdoc: {
+        expression: "[AST_Node?] the value returned or thrown by this statement; could be null (representing undefined) but only when is_star is set to false",
+        is_star: "[Boolean] Whether this is a yield or yield* statement"
+    },
+    _walk: function(visitor) {
+        return visitor._visit(this, this.expression && function(){
+            this.expression._walk(visitor);
+        });
+    }
+});
+
 /* -----[ TreeWalker ]----- */
 
 function TreeWalker(callback) {