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/output.js

@@ -560,6 +560,20 @@ function OutputStream(options) {
         }
     });
 
+    PARENS(AST_Yield, function(output){
+        var p = output.parent();
+        // (yield 1) + (yield 2)
+        // a = yield 3
+        if (p instanceof AST_Binary && p.operator !== "=")
+            return true;
+        // (yield 1) ? yield 2 : yield 3
+        if (p instanceof AST_Conditional && p.condition === this)
+            return true;
+        // -(yield 4)
+        if (p instanceof AST_Unary)
+            return true;
+    });
+
     PARENS(AST_PropAccess, function(output){
         var p = output.parent();
         if (p instanceof AST_New && p.expression === this) {
@@ -868,6 +882,9 @@ function OutputStream(options) {
             output.print("static");
             output.space();
         }
+        if (self.is_generator) {
+            output.print("*");
+        }
         self._do_print(output, true /* do not print "function" */);
     });
 
@@ -887,6 +904,17 @@ function OutputStream(options) {
         self._do_print(output, "throw");
     });
 
+    /* -----[ yield ]----- */
+
+    DEFPRINT(AST_Yield, function(self, output){
+        var star = self.is_star ? "*" : "";
+        output.print("yield" + star);
+        if (self.expression) {
+            output.space();
+            self.expression.print(output);
+        }
+    });
+
     /* -----[ loop control ]----- */
     AST_LoopControl.DEFMETHOD("_do_print", function(output, kind){
         output.print(kind);
@@ -1218,13 +1246,8 @@ function OutputStream(options) {
         output.print(self.operator);
     });
     DEFPRINT(AST_Binary, function(self, output){
-        var isYield = (self.left.operator == "yield" || self.left.operator === "yield*");
         var op = self.operator;
-
-        isYield && output.print("(");
         self.left.print(output);
-        isYield && output.print(")");
-
         if (op[0] == ">" /* ">>" ">>>" ">" ">=" */
             && self.left instanceof AST_UnaryPostfix
             && self.left.operator == "--") {
@@ -1234,10 +1257,7 @@ function OutputStream(options) {
             // the space is optional depending on "beautify"
             output.space();
         }
-
-        isYield = (self.right.operator == "yield" || self.right.operator === "yield*");
         output.print(op);
-
         if ((op == "<" || op == "<<")
             && self.right instanceof AST_UnaryPrefix
             && self.right.operator == "!"