Disallow extension properties with backing fields

#KT-1682 Fixed

js/js.libraries/src/core/string.kt

@@ -36,7 +36,7 @@ native public fun String.match(regex : String) : Array<String> = js.noImpl
 native public fun String.trim() : String = js.noImpl
 
 native("length")
-public val CharSequence.size: Int = js.noImpl
+public val CharSequence.size: Int get() = js.noImpl
 
 library
 public fun CharSequence.length(): Int = js.noImpl

js/js.libraries/src/stdlib/domCode.kt

@@ -7,7 +7,7 @@ public fun createDocument(): Document {
     return js.dom.html.document.implementation.createDocument(null, null, null)
 }
 
-native public val Node.outerHTML: String = js.noImpl
+native public val Node.outerHTML: String get() = js.noImpl
 
 /** Converts the node to an XML String */
 public fun Node.toXmlString(): String = this.outerHTML