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.

test/mocha/object.js

@@ -0,0 +1,20 @@
+var Uglify = require("../../");
+var assert = require("assert");
+
+describe("Object", function() {
+    it ("Should allow objects to have a methodDefinition as property", function() {
+        var code = "var a = {test() {return true;}}";
+        assert.equal(Uglify.minify(code, {fromString: true}).code, "var a={test(){return!0}};");
+    });
+
+    it ("Should not allow objects to use static keywords like in classes", function() {
+        var code = "{static test() {}}";
+        var parse = function() {
+            Uglify.parse(code);
+        }
+        var expect = function(e) {
+            return e instanceof Uglify.JS_Parse_Error;
+        }
+        assert.throws(parse, expect);
+    });
+});