[WIP][NotForLand] IDL dictionary support

Source/core/dom/SomeDictionary.idl

+// NOT FOR LAND -- this is an example of IDL dictionary.
+
+[
+    GarbageCollected
+] dictionary SomeDictionary {
+    long a;  // Example of a primitive member
+    long b = 2;  // Example of a primitive member with default value
+    DOMString stringMember = "doraemon";  // Example of a string member
+    Element element = null;  // Example of an interface member
+    boolean? nullableBooleanMember = null;  // Exmaple of a nullable member
+    sequence<DOMString> stringSequenceMember;  // Example of a sequence member
+    Element[] elementArrayMember;  // Example of an array member
+};