Implement index operator super call

tests/language/src/SuperOperatorTest.dart

Index: tests/language/src/SuperOperatorTest.dart
===================================================================
--- tests/language/src/SuperOperatorTest.dart	(revision 358)
+++ tests/language/src/SuperOperatorTest.dart	(working copy)
@@ -6,25 +6,57 @@
 
 class A {
   String val = "";
+  List things;
+
+  A() : things = ['D', 'a', 'r', 't', 42];
+
   operator + (String s) {
     val = val + s;
     return this;
   }
+
+  operator [] (i) {
+    return things[i];
+  }
+
+  operator []= (i, val) {
+    return things[i] = val;
+  }
 }
 
+
 class B extends A {
   operator + (String s) {
-    super + (s + s);
+    super + (s + s);  // Call A.operator+(this, s + s).
     return this;
   }
+
+  operator [] (i) {
+    var temp = super[i];
+    return temp + temp;
+  }
+
+  operator []= (i, val) {
+    // Make sure the index expression is only evaluated
+    // once in the presence of a compound assignment.
+    return super[i++] += val;
+  }
+
 }
 
 main () {
   var a = new A();
-  a = a + "William";
+  a = a + "William";  // operator + of class A.
   Expect.equals("William", a.val);
+  Expect.equals("r", a[2]);  // operator [] of class A.
 
   a = new B();
-  a += "Tell";
+  a += "Tell";   // operator + of class B.
   Expect.equals("TellTell", a.val);
+  Expect.equals("rr", a[2]);  // operator [] of class B.
+
+  a[4] = 1;  // operator []= of class B.
+  Expect.equals(43, a.things[4]);
+  Expect.equals(86, a[4]);
+
 }