Fixed ret by var in js

tests/varres/tvarres0.nim

@@ -0,0 +1,30 @@
+discard """
+  output: '''123
+1234
+123
+1234
+12345
+'''
+"""
+
+# Test simple type
+var a = 123
+proc getA(): var int = a
+
+echo getA()
+
+getA() = 1234
+echo getA()
+
+
+# Test object type
+type Foo = object
+    a: int
+var f: Foo
+f.a = 123
+proc getF(): var Foo = f
+echo getF().a
+getF().a = 1234
+echo getF().a
+getF() = Foo(a: 12345)
+echo getF().a